I’m afraid that won’t be acceptable to a mortgage lender MOnica and Gaga. They always insist on being repaid first if the property is sold.
It seems it’s too late to back out and OP just has to do her best to make things work out, though I do feel she’s perhaps not well enough to do much childcare. The post which was unearthed from 2020 suggests that one of the children isn’t much more than a baby. I anticipate that she will be expected/have agreed to contribute to household bills - fuel, food etc - so not much of her pension left, and help out around the house. It really isn’t a great situation and OP should have thought about all the downsides before. There are upsides though, in terms of being with her daughter and grandchildren, and she must now focus on those, try to join some groups to enrich her life and make new friends to the extent that childcare will permit, and basically do all she can to make it work for everyone. There’s nothing to be achieved now by focusing on the ‘what ifs’.
Son on rebound - wants to marry