I have horrible flashbacks to when I worked in retail in high school, and this awful 12-13 year old (who must have been babysitting a brother) was yanking a 3-4 year old around, jerking on it like someone would to a "bad dog"
However, if you are using them in an appropriate, loving way, I think they're totally fine. Plus from a provider standpoint, you only have two hands and many more kids than that. It's either a backpack or let them wander, which is worse?