I own another house to the one myself and DH live in and own together. It was my grandparents house and was left in trust for me. I inherited this before me and H married and it is very much my house and all protected etc etc.
Currently this house is rented out and my plan has always been that eventually when my DC has grown up, I will sell it and give the proceeds (or the majority at least) to them to give them a step up, the house is currently in a trust for DC in case anything happens to me but it still gives me full control / ability to sell etc.. DH knows this.
Me and DH also own a house together that we bought during our relationship and is the family home. Here there is me, DH, our child and my stepchildren, DHs older sons. The children are all too young currently for this to even be an issue right now but a hypothetical conversation stirred up a bit of a tiff between me and H last night.
Basically we were talking generally about young ones these days and how hard it can be and we got onto the conversation of wondering how long kids in general will end up living at home for nowadays (SC currently here about 30% of the week). DH then said well we could always let SC rent the other house when they want to move out and how nice it is to have a house there that means we can offer a home to the children at a lower cost and so on... for context SS's are 15 and 17 and our child is only 5 so it's obviously more likely to be SC living independently quite some time before our child.
Essentially I said well no? That house was already rented and it is there for benefit of DC, not to he used at a cheaper rate for SC, didn't say it quite so bluntly but that was the gist that basically it was DCs house, not a "family" house and that until DC needed it to either live in or sell for the proceeds it would be rented as normal.
This got a bit heated, but there are a few reasons I wouldn't be open to this suggestion and the main one is I think it's a slippery slope. So say we offered it to SC, bearing in mind still this is all entirely hypothetical, but say we did, and they or one of them lives there for many years paying minimal rent, I don't believe that it wouldn't begin to feel like a "shared" house, like we either had to continue to let whoever it were live there and not give it to DC as planned or we'd have to share the proceeds. I just think it opens it up to getting messy and I'd rather keep it simple and separate. It's DCs future, not a family house that can just be used by whoever.
I've suggested in the past that DH start thinking about putting together some sort of savings or whatever for SDC, he could as the money isn't the problem, but he never has.