I have an individual contract for each family detailing everything. I include a section describing the share situation and specifics to that. You should place a clause stating that the families writing one check is responsible for each other's payment. So if one family doesn't pay, you still get the full payment.

I would have a contract for each check you will recieve, making clear to the families that will write one check that they are all responsible for seeing that you recieve full payment each paycheck. There will be no "well here's our half".

Sorry B&C will be paying me together. One cheque from them one from family A

Do you have a contract with each separate family? I am currently in a share with 3 families. Family A is full time and will pay me a flat rate per week. Family B&C are friends and need me alternating days. They will be paying me together. Do I create a contract for both families separately? How do you do it?