Kids on Bikes
What you need to ride with kid(s)
a bicycle (duh)
kid-sized bike helmet --> our first kid liked a Lazer helmet (the exact model, which had great magnetic clasps, was sadly discontinued but they make other good ones, like Lil'Gekko for toddlers) experts like Twowheelingtots and NY Magazine recommend the Giro Scamp and others...consider MIPS for extra protection. For our 2nd kid we got the Giro Scamp, which seems great so far.
child seat of some kind --> most popular is a Yepp seat (also recommended by Wirecutter), we got the Yepp Nexxt Maxi but the original Yepp Maxi is also highly recommended (both are designed for 9 months to 6 years or 40 pounds); they fit on most bikes' rear racks, if you don't have one we liked the straightforward Topeak Explorer rack (in SF, The Bike Connection in SoMa carries both)
Riding with infants?
Many doctors advise waiting until ~9 months to bike with a child, due to concerns that bumps in the road could be bad for infants...we didn't bike with our older kid until he was over a year old, but with our younger one we started around 9-10 months...and apparently in the Netherlands it's common to bike with very young infants (often in a car seat). It seems a bit of a double standard since we generally don't think twice about letting infants go in a car down a dirt country road...? I'd probably be comfortable with taking a young baby on smooth asphalt, especially if the bike has good shocks.
Cargo (e-)bikes & kids
Almost any normal bike can carry a kid with a Yepp seat, but you may decide to upgrade for the same reasons we did:
less tipsy - as your kid gets heavier, they can upset the balance on a normal bike (especially at low speeds)
easier hills - San Francisco has some gnarly hills, some of which are tough with a kid in the back
faster/longer range - electric assist lets you go faster/longer without breaking a sweat (especially helpful when dropping off kids at school on your way to work)
more storage - cargo bikes let you bring a kid and groceries (or big toys like scooters) on a single trip
Models to check out
Tern GSD - the bike we have (see photo above and below), the smallest bike that can carry two kids, has smaller (20") wheels so it isn't tipsy (since your kid is closer to the ground) and it's as long as a normal bike despite being able to haul a lot of stuff; the compact SUV of bikes...some people don't like the touchy steering (due to the small wheels) but we like it. Great for city living. Very popular now in San Francisco, and for good reason. A fully-loaded one costs ~$5K but it's well-made and can replace a much more expensive car.
Tern HSD - smaller/lighter/cheaper version of GSD, only carries 1 kid but otherwise has similar design
Tern Quick Haul - very similar size/design to the HSD, but cheaper (both in price & components, see comparison here)...even cheaper is the Short Haul which has a similar design but no e-assist (might be good for families with 1 kid and no hills)
Benno Boost - another relatively-small bike that can carry two kids; compared to the GSD it feels more like a "normal bike" (due to 24" tires) but it doesn't carry quite as much, also has a >20 MPH option. Along with the GSD, very popular at our preschool.
Benno Remidemi - only carries 1 kid but looks like a lot of fun, a bit like a Vespa! ...20" wheels (so similar pros/cons to the GSD/HSD, although Remidemi's are a bit wider), can attach panniers/racks for storage (slightly shorter rear rack than the HSD)
Specialized Globe Haul - also has 20" wheels, the ST version carries 1 kid and the LT version carries 2 kids (basically the ST competes with the Tern HSD and the LT with the GSD)...Wired likes this one
REI Co-op Cycles Generation e1.1 and e1.2 - 20" wheels, carries 1 kid, could be a cheaper alternative to the Quick Haul/Remidemi/Globe Haul ST
Xtracycle - various "longtail" e-bikes that carry 2 (or even 3!) kids, good if you have more space and want to haul a lot of kids and/or stuff; Yuba is another popular "longtail" brand
Radpower - various cargo e-bikes that are cheaper than the others listed here...they're popular in SF but I've heard that they need more frequent maintenance (you get what you pay for?); might be a good option if you're particularly handy
Blix Packa - looks like a GSD knockoff with slightly larger tires; looks worth investigating but could be "cheap" in both senses of the word? (similar is the Aventon Abound Ebike, which Wired recommends as a budget option)
Box bikes - this style of bike is very popular in the Netherlands, so much so that they are a political constituency ("box bike mothers" are Dutch equivalents of US "soccer moms"), very comfortable/stable for kids, hauls a lot of stuff, the nicest ones are made by Riese & Müller but there are various other manufacturers including Urban Arrow...we tried these out but thought they felt too big/cumbersome (both to ride & store). Madsen makes some bikes with the box in the back...? These bikes are probably a good fit for suburban families with a garage. Also great for dog owners.
Shops, groups & accessories
The New Wheel - awesome electric bike shop in San Francisco and Larkspur (Marin), very knowledgeable and helpful; they let you do test rides with your kids...their bikes are pretty pricey but they only carry high-quality models that should last you a long time. Their YouTube channel is also helpful for learning about e-bikes.
Rolling Jackass Kickstand - this has an awful name and costs a lot, but if you can afford it, it's worth it if you're regularly transporting kids—it makes the bike super-stable, so I feel comfortable with Adam climbing on/off the bike on his own --> Note: the new "Gen 2" 2021 Tern GSD has an "Atlas" kickstand that supposedly is just as good
San Francisco Bicycle Coalition - if you live in San Francisco, you should join this group that advocates for safer bicycle infrastructure across the City—they do some great work
Safe Routes & Safety
Sometimes other parents ask me if I feel safe riding on a bike with my kids. I do...but that's largely because I'm careful about how I bike and what routes I take.
Some general advice on safe biking:
Low-traffic streets - I tend to favor lower-traffic, narrower streets where cars don't drive as fast. For example, when heading downtown with my kid, I avoid Folsom Street (which despite having a protected bike lane has a bunch of fast-moving cars), and instead take Townsend Street (which has a partially-protected bike lane, but also less & slower-moving traffic). Similarly for north-south travel in the Mission I prefer Shotwell or Harrison to Valencia or Folsom.
Slow Streets & Bike Lanes - Protected lanes are better than unprotected lanes (or none at all), but also consider the network of Slow Streets, which tend to be low-traffic and bike-friendly. To see bike routes, the SFMTA has a map, and Google Maps isn't too bad. Sometimes I'll explore an area on a bike without my kid before taking him there.
Avoid Doors - Don't bike too close to parked cars, lest you get "doored" (run into someone opening a door unexpectedly)
Signal & Be Nice to Drivers - Most drivers are not actively bad/mean, but they sometimes forget to look for bikes, so bike defensively. Get a good bell & use it to make sure they know you're there, and use hand signals when you're turning. A little bit of kindness and a smile can go a long way, too.
Worst Case Scenario: Walk - You can always walk your bike. Walking a few blocks has gotten me out of a few pickles where I ended up on an unforgiving street with lots of fast-moving car traffic and no bike infrastructure.
...that being said, I live in a part of San Francisco that has pretty decent bike infrastructure (at least compared to most of North America); you have to evaluate your community/neighborhood yourself