It my understanding that you can be in support bubble with your partner and a childcare bubble to look after the grandchildren. However, you shouldn’t be mixing with your daughter and son in law indoors so they should be just dropping the children off at the door etc. Re your parents - you are able to give care, but bear in mind the more people you mix with, the greater the chance they could be inadvertently exposed to Covid.
Not worth arguing with the police, too expensive!! Any idea who reported you?
There is no support for carers in the current bubble system. I am a single carer and care for S who has autism and a related learning disability. He can only hold simple conversations (like a very young child.) Unlike a parent who could be living with nearly adult children, I am unable to form a bubble.