Have you offered this to your other children? Living rent free for minimum a year because let's be real here, cost of living is going up. If they are having a baby, they won't be saving much soon to move out in a hurry. You can say your other kids have a house now but was it offered to them when they were starting out or at least they were offered something financially equivalent?
You say you aren't in a position to offer childcare but you would definitely be caring for your grandchild if you live under one roof, at night if not during the day.
I'm saying this because it creates tension in the wider family if not all children are offered the same opportunity. It will become evident really fast to your other children who your favourite child is.
Seeing it in the family, often the responsible children gets overlooked just because they are responsible. They don't need your help because "they have a house already".
How your other kids and their partners will see it is you offering this son a financial leg up, plus supporting his family post partum and being much more accessible to babysit for them. You are posting here to do everything to make it work for them to stay.
After considering all this on what this will impact on your other children;
- Set a specific time they need to move out. Else a year can be turned to several and before you know if they are expecting to inherit the place.
- They need to pay at least token rent to cover utilities and common groceries.
- If they aren't paying you market rent, have set chores/house projects they need to do for you in this time. This could be yard maintenance, painting etc. Effectively do things to help around the house to contribute in return for the financial help.
- Your son needs to come up with a back up plan if it doesn't work out.
- It's weird to be cooking separately all the time under the same house especially as a family. Set up a schedule for a few times a week being family dinner night and who's rostered for buying the groceries and cooking. I'm saying this because my SIL lives with my MIL and she takes my MIL's credit card to do groceries for family dinners when she is living effectively rent free in her home. It's poor form for an adult to be taking so much advantage of elderly parents and trust me the rest of the family is acutely aware.
On the side note, also be prepared that your other children and their kids will be visiting you less for holidays since your house no longer have the space. Your other grand children might also wonder why their granny don't provide the same support to them unless you make up a plan on how to share your time and resources fairly.