I wouldn't be happy, either. But then I wouldn't let DD go away for a five day visit unless I was absolutely sure of what would be happening. GPs don't automatically have overnight rights unless they've earned them in my opinion.
If you mean that your DD has had a fake suntan put on, that's bang out of order. You probably don't want her to become sophisticated and image conscious so quickly. It should have been checked with you first, especially as the product is very drying on a child's skin. The nail varnish I wouldn't take issue with.
I don't know that I'd believe her re custard creams and strawberry mousse for lunch. I would check that with PIL. If it's true, it's not on. Not for a five day period. It's not fair to your DD. If you want her to go on a low sugar diet then that is what should happen, although I agree that a little bit of spoiling is par for the course. Sugar highs aren't a myth as anyone who has fed a child skittles will know.
I would ask for more information from PIL regarding the comment that she should always tell the truth. It could be completely fine but it would be worth inquiring if this had been said because it was thought DD had lied. I would also check that her story about not having been outside is accurate because children do need exercise every day.
Children need stability, good food and a healthy lifestyle. If they're going it be spoiled, it should be done in a way that doesn't leave them worse off - cooped up on a sugar high being decorated like a doll is not in a child's best interests. Grandparents may like to spoil, but they need to balance that against responsible, kind treatment.