Should we downsize to help our adult children out?

You would probably have to move area. The national average for a semi is about 400k.
If you are not able to get a place in the area you are in and you I would stay put. However if there is a cheaper alternative in a place you have all the amenities in I would soendcsone yime there and if it was good I would move. Makes things easier later on. I should have moved years ago, but with a diffucult build next door think I left it too late.

‘ Bad granny’ has surfaced yet again! I would be looking after myself and my future, I’m afraid. We don’t know how things are going to turn out, and it’s a giant step to take with fingers crossed that everything will be okay. Of course it could be, but what if your children decide to move away for whatever reason, or relationships may break down, or jobs could be lost or improved, so many things can happen.
I may think differently if a child was in serious difficulty, but, for me, I wouldn’t be jeopardising my own comfort in the future because they could do with a bit of help. I’m sure many of us could have in our early years of marriage, but we got there in the end!

Although we struggled financially when we were young I would have hated my parents giving up their home to help me out.
They were happy there and had put a lot into that house as I'm sure you have done in yours.
I don't think it's a good idea to give up your home for that reason.

What are prices like near you and what could you afford if you did sell up and give DC some money? I know one couple who did this but they moved to a rural location from London where property was much cheaper. Our house is worth pretty much the same as yours as we would have to move areas if you we did what you suggest.