that there is absolutely no excuse for you hitting someone with your car when they are on a pavement. That's exclusively on you
That's insane.
There's joint responsibility.
For example, I'm a runner. If I'm on the pavement, I watch for cars coming out of driveways & prepare to stop.
I'm not Usain Bolt or anything but if I ran full tilt down a road, not checking, of course it would be my responsibility if a car hit me. They can have checked all round but me suddenly appearing would not be their fault.
Likewise, if a driver does shoot out of a drive, without checking and moving v slowly, then they are being irresponsible. This happens more than it should.
As a consequence I'm extremely wary, and will stop if I think a car might be moving out.
Everyone needs to take responsibility. In this case, OP (who has done), the parent of the child, who even seeing a neighbour get into their car, should have checked their child's position.
People keep talking about legal responsibility and I get it. But in practical common sense terms, it makes sense for everyone on roads and where there's traffic to be sensible, protect themselves and others.
Sadly I'm aware of more than one case where a child has been killed by a vehicle on a driveway. There have been no prosecutions as horrifically, it's usually the child's parent, or at least a close relative.
The law isn't the focus here. It's careful cautious action - which includes closer supervision by the parent here, and the safest driving by OP, which she did, tho I'd agree with reversing in, if possible.