hah thanks @joeyduck for the tag.

regarding trailers, i would read this: Carrying Children on Bikes - Trailer versus Kid Seat

i agree the most you have to worry about really is the uneven roads/pavement. with a trailer you are "driving a bus" and have to make wide turns and such. just being aware of your entire bike outfit. trailers are a great solution for parents who are working out. i personally think it is far safer to ride with the kids behind than in front, but hey every parents' perception is different and there is no real data to extrapolate anything else. just find something you and your kid(s) like riding and feel comfortable with and you should be set.

we had a trailer from age 1-4 and we stopped using it at age 4 b/c we won a cargo bike in a raffle, local here to NYC. the trailer is a perfect low cost option. (side: ours was high cost bc it transforms into a jogging stroller as well as a sled - we do a lot of XC skiing). however at higher weights, it was hard, not going to lie. 27 lbs trailer plus 45 lbs of kid plus other misc. weight - with my cross bike on hills i often had to gear down. i previously had a bike with a triple chainring - that works really perfectly for towing higher weights b/c you can gear down a ton, which helps a lot on hills.

the flag is fine as an alert to drivers. in NYC there are cyclists everywhere and drivers are pretty much always aware of us. wearing bright colors, flash bright lights, some even outfit their bikes with christmas lights. so, anything to be more visible is preferable.

ride predictably in traffic -> you will be fine for the most part. that said, there are bad drivers out there. you'll just have to be highly alert of them and steer clear of them. if you are FAR from them, you'll be OK.

view all child seats and trailers, as well as cycle-trailers (and other child bikey things!) here: Bike Trailers: Comparison Charts ? Two Wheeling Tots <= several someones on bikeforums.net recommended this site and i can't recommend enough as well. when in doubt, check out if Natalie has reviewed it.

OP @woodycooks, get yourself to a local shop that sells and services cargo bikes, and test ride some. most likely you will walk out with a midtail or longtail, probably the xtracycle edgerunner. they tend to outsell all other cargo bikes, something like a 10 or 15 to 1 ratio here in NYC.

we ride in a yuba mundo and we love it. great for groceries, rides to the various parks, and beaches. riding opens a ton of opportunities up.

EDIT: i just read the rest of what you wrote - we are planning to cart our school-aged kid to school this fall, and my husband is looking to add a bobike junior to the back of his bike bc he doesn't want to ride the yuba mundo to work everyday. (he has the surly crosscheck too, and it's about 24 lbs vs the 45 lbs a yuba mundo weighs). the school* is about 1-2 miles from home and work is bout 6-7 miles from the school. so for family bike rides in the neighborhood, we'll ride the yuba mundo. for a quick dropoff near home then a longer commute in to work, we're going with the bobike junior. i have no idea how it will work but we've seen other families do this and after speaking to them, they seem happy with it - so we're copying and hoping for the best.

*really should be potential school - we are actually still waiting to hear back about our school placements so when we know for sure we'll go get the bobike junior.