WOW! Toughie.
We always pack a bottle of sun cream in our boys bag when he goes to the child minder, and if she had sworn that she’d put it on we’d trust her that she had, but then she’s not your child minder – only you can make that call.
I think we’d take our boy to the GP’s just as a check – if when questioned how it happened we’d be forced to reveal the situation, and I think that’s where the proper authorities/procedures would kick into action if required/deemed necessary.
Neglect is tricky – if she’s stated that she did put it on, how can you prove otherwise? One could argue that reddening cheeks on a child that age could be seen as symptoms of teething, or heat, wind (blowy stuff not bottom burps) – and reddening doesn’t always show up straight away.
Just playing Devil’s advocate here.
You sound like a sensible chap, I’m sure you’ll do what’s right by your boy.
Good luck, whatever you decide.