One grandchild has refused to meet, and the other twin won't meet without the other. A delicate matter, but it is good that your Daughter in law who may be traumatised by your son's behaviour as much as her children, is prepared to maintain contact with you.
I would tentatively suggest an 'accidental 'encounter in the Park or open space so that the children may run away and play if they wish and she may keep moving likewise.
I would not mention it to your son as he may use it as an excuse to make contact. The fact that he has traumatised his children is serious, and is endorsed by his behaviour to you, making you take great care not to upset him, in your own home. Of course you love him, but you must not allow him to traumatise you too, which is what he is doing.