@chickadeeeee
Before marriage I would have something in place.
If I died first my house would be left to my children as would any money, although he would get to live his life out in the house providing he was single or not remarrying.
I have a will but would obviously need to update it.
I have considered changing my house into my children's names.
I have known him 9 years. He is working now and financially secure as in day to day living. No savings or assets.
It's just this debt that bothers me.
But OP, you could drop down dead the day after making the will, and he might live to be 99. He might have nothing to do with your children in all those years. That's their inheritance, not his.
And I wouldn't change your house into your children's names - again, you don't know what's going to happen in the future. It's your house and you need to always have that control over it.
If you do want to stay with this guy, then just don't get married. Write a will leaving everything to the children. Mention him in the will, giving him the right to stay there for a while, eg a year or two, with him being responsible for any upkeep while he lives there.
Marriage is romantic for the day but you need to think of it as a business agreement. Would you set up in business with someone who owed £20K? No matter what he says, that debt didn't get there without some of his involvement. The debt is 12 years old and he hasn't done anything about repaying it until recently. Would you put your house and your money into a legal partnership with someone like that, knowing that they could legally use your money?
Forget the romance - that should be in everyday events. When you have a house, money and children then you can't afford to legally tie yourself to someone who has such a bad financial track record. (And an absolutely appalling track record of paying his debts.)
Let him live there rent-free if you like. That's an amazing gift for anyone. But that should be your limit.