Thanks for the advice.
I do make sure every time that they've asked their parents, and the parents do know us, since we have kids the same age. (It is basically an extended family all living in the same apartment house.) They all hang out a lot around their side porch, and are often present when I come over to work on the bikes. In fact, the bike work started with them asking me about changing a tire. I'm pretty sure none the adults have bikes, so I can't ask them to ride with us. Left to just the parents, the kids are limited in their ability to ride except for the parking lot across the street in the evening, and would probably lack education and practice in good riding habits.
I haven't yet taken the big group outside the neighborhood streets, and probably won't for a good while. I stress riding on the right, or the sidewalk, paying attention to stop signs, and if any of them got ahead, waiting for me at the next intersection. I'll definitely be careful about buying them things. I wouldn't buy them bike things unasked, but I have been trying to get them to know that there's a bike store just a few blocks away that they could go to. I suspect that they believe the bike store will be too expensive, for both purchases and repair.