Its very unlikely to be a pit bull, it is very likely to be a staffy cross. Staffies are great with kids, that does not mean I would trust one with small children, if that dog should turn nasty it would cause so much damage, these dogs are immensely powerful for their size, they can also be a bit touchy around other dogs. The RSPCA are about as useless as anybody else at identifying a cross breed IMO.
I know my dogs would never ever deliberately harm any person or creature, its their nature, unless in defence of me or my good lady or family or if really provoked to defend theselves, if they get attacked they just want to escape, they never want trouble. However, I would not leave any of my dogs with small children, simply because if they stood on their bare foot or hand they would cause damage or if they knocked them over, they weigh in at over 1o stone each.