bluebear - I guess that any of that is OK (although not something I would do as a childminder) if it is something agreed to by the parents.
I don't like the idea of leaving a child outside locked school gates, I assume that there isn't a staff member there? Perhaps another minder or parent is around? I know of minders who literally drop children off on pavement and watch them run into playground, but it is something that the parents are fine about.
Do the parents know the minder isn't staying with the 2nd child? If they are happy with the arrangement, then I guess it's OK. (No, I wouldn't do it myself, I stay with children myself until they go in).
If you have a problem finding minders to do school runs, I guess parents take what they can get. I hope that the parents know that this is the arrangement.