Just make sure you do your homework beforehand to see what classifies you as a resident.

She has not been to work nor has taken her kids to school.

This is more likely a family war of abandoned children as he has never mentioned any other connection to the kids. but only that he doesn't owe the "girlfriends son" anything, and that his former girlfriend and "her son" hate him.

I'd first ask your kids or grandkids if any of them know how to build a basic website (if you have a family member that is a designer of some type the probably do).

Grandma is the best, safest and cheapest sitter for the kids while their on school break.

The cool kids want to live here.

Do some good homework and speak with both professionals who offer these services and your father's licensed tax advisor before moving forward with any plan.

If the kid is older I'd watch things closer and if they are causing issues I'd give them notice based on the kid violating lease terms if they are doing bad things. 

Cardboard is good for sledding.... my husband used to do so as a kid, at a small hill in the neighborhood that the kids dubbed "cardboard hill".

He told the college kid that worked for him just to charge me the same as the previous year.