You do have a point with that. I think it goes both on that one. There are enough times parents let their young ones roam unattended, sometimes those parents get seriously upset at other people when an incident occurs. I think the issue might be the queues are not the best place to allow that, many twists, turns, most using just a chain (so a young child can easily get away), some are very dark queues, etc. It's as much as safety hazard there. But there def. places where kids should be able to walk around, closely monitored by their parent/guardian to have fun without creating a safety risk.
I've been behind touring groups that let their kids walk around and it's usually not a huge deal but when the kid doesn't move for a while and you're not trying to interfere it puts you in an awkward position of "do I say something nice and sweet to the young child?" "do I not?". Then there are queues where you are told to fill all the space and timing can get really messed up like HM. When we were in DLR in 2019 I remember the cast members getting quite annoyed at guests who wouldn't keep moving and yet were complaining about the length of time in line. Adults are capable of slowing down queues too but a young child adds in they can trip, fall, get lost by running ahead or falling back behind, etc.