We ride on the sidewalks outside of our neighborhood, but on the sidewalks when we're on busy roads. The sidewalks here are usually right next to the street, and the grass side isn't always level with the sidewalk. I wouldn't want to ride anything wider than a regular bike on our sidewalks, but that's because no matter what time we ride, there are always people out walking or riding here, so we have to maneuver around pedestrians, or come to a stop while they go past. It seems like the sidecar would make it impossible to maneuver around pedestrians.
What I meant about locking up the bikes is that good locks are heavy and it's hard to find a place on a kid's bike frame to attach the lock where it will be out of the way when they're riding. So we ended up putting the locks in the bike bags that come on the kids bikes, and then the bike bags ripped. Now they carry the locks in their backpacks on the way to school. I'm planning to get a basket for my bike and I guess that will help with where to put all those locks while we're riding.
Our experience with department store bikes is that they're fine as long as you're an occasional rider. I'm still riding one because that's what my budget allows for, but I bought it in May and things keep wearing out on it. I've already had to replace the brake cables and I've broken two sets of pedals, and had an issue with the back wheel getting shaky. I do my own repairs, but if I were taking it to be repaired somewhere, it wouldn't be long before I spent more than the bike was worth. My daughter had the same problem with her bike...it was fine for riding around the block once a week or so, but once we started riding five miles a day, stuff started breaking. I found her a better bike on Craigslist, which is actually a good place to look for bikes.