@River9871
Think DP needs to speak to a solicitor to confirm how to do this, but then explain that you can: a) buy a joint property as tenants in common, so that both your halves/shares are protected for your respective children; b) reassure her that as you are not married you would have no claim on any inheritance he would receive from her; and c) that if you do marry at some stage, you could have a prenup where both your individual assets are ring-fenced for your own children so anything she wishes to leave him for his lifetime benefit but eventual passing on to her GC would be protected.
Obviously, he could have considerable frailties in his old age and end up using his inheritance up by paying for nursing care and the GC may get nothing anyway, but if this is a concern, she may then wish to leave her worldly goods directly to the GCs anyway. Given how hard it is for young people to get on the property ladder and the fact that he will have a nice home with you, OP, this may actually be the best option anyway, especially as it removes any risk of paying inheritance tax on it twice (when she dies, and then when he dies).
You can change the tenancy in common to a joint tenancy should you get married, but you may both want to continue to protect your interest in the joint property for your respective children, so an alternative is to retain the TiC and then leave each share in a trust to each of your children with the surviving partner having a lifetime interest (ie, surviving partner can stay in the joint house if they wish and the inheritance of all children can only be realised upon death of the second party, or should the surviving partner choose to sell/downsize prior to their death).
I am a little out of touch on land law and inheritance law, but DP can simply go to see a solicitor together and arrange things in a way that gives her peace of mind. The very elderly of that generation have very fixed ideas about this stuff so it’s best to set it up in way that takes the heat out of the situation for you and DP going forward.