My thoughts, taking the issues separately rather than the whole picture......
I don’t let children in my care go to school on bikes or scooters. I’m sure some would be perfectly safe, but I’m very conscious that it freaks out some other pedestrians who have bad experiences with less careful children.
Whether these particular children with your childminder are under control depends entirely on the children and the childminder. So it’s not for me or any other MNer to judge.
In terms of control/behaviour, the scooters are a red herring. They can just as easily dash off on foot as scoot away. I defy any adult to maintain control if two wilful children decide to run off in opposite directions.
The childminder can’t be blamed for stupid drivers and selfish parking (which IME is the norm on school runs, but that’s a whole new thread.)
Your son's safety on the school run should depend chiefly on how well you've taught him his Road sense. It’s not something parents should think they can outsource to a childminder. If he knows to stay on the pavement, then he's safe. If he thinks it’s ok to run into the road, then he's at risk no matter how good the childminder. What the other children are up to makes only minimal difference.
As mentioned, a childminder can normally care for up to six under-8s, of which up to 3 can be of pre-school age. Exceptions are allowed for the pre-school age group, but the six under-8s is pretty much set in stone.