Interesting to see the responses. Some were not specific about how much $$ they provided, some disclosed amounts more akin to a car loan. And some just objected to any parental influence on found money for their offspring.
The OP asked about a few different scenarios for a kid's pre-nup - house down payment or similar and future inheritance. Those are different things to me, plus there is a materiality threshold I would consider.
Easiest first - for a future inheritance, no I wouldn't demand a pre-nup. Too many unknowns. My age at death, how much I've spent on hookers, blow and home health care on my way out the door
. My kids understand what happens if I leave tomorrow, and they're both too prudent/frugal to just give it up if their relationships go south. They also have good advisors they have known for years to call on. Plus, I'll be gone, and if I can care, there won't be anything I can do about it.
Along the way while I'm alive, different approach, and here's where the materiality threshold comes in. If I give $10K to a new in-law to pay off student loans when their money is otherwise managed prudently, not going to worry about what might happen down the road. Clean start as a young couple, etc. OTOH, if that gift still leaves $10K on their credit card debt, not doing it. Pay off the credit cards first, then come to pappa.
Moving up the materiality ladder, if I give $50K or more for a house down payment, I'm going to make sure my kid gets the benefit of that if things go bad. That amount would be an advance on their inheritance, and I will influence that as long as I can. Doesn't have to be a pre-nup (with all the drama that entails), but it needs to be clear, and legal.