I am heartily sorry for you and your friend too and agree that his children are behaving immaturely and unkindly, but I would like to put forward some points for consideration.
How long ago did your friend's wife die? And did the son and his father who both witnessed the accident receive any psycological help? Can you be entirely sure that the children and their spouses are not still acting partialy due to the trauma of the death?
Another point: the media are full of stories of this kind, but also full of stories regarding the difficulties of inheritance complicated by a parent's second marriage if the new spouse outlives the one who had children.
As I understand the law in England a man or woman who remairries is not obliged to settle the desceased spouse's estate with the children of that marriage before re-marrying, as he would be in a lot of other countries, and the testator can still leave his entire estate as he wishes - there is no compulory amount that must be left to the children of his marriage or marriages.
If my understanding is correct, I can well understand the concern shown, although not the manner of it, by this man's children. They risk their father disposing of property and goods and chattels that had actually belonged to their mother without their knowledge or consent.
I cannot pretend to tell you what you can or should do now. (But am taking a shot at it all the same, sorry!) That depends entirely on your feelings for this man and his for you. If he has broken up with you basically because his children are more important to him than you are, then the sooner you force yourself to move on the better.
However, it sounds as if he does love you, but finds himself between the devil and the deep blue sea. Could you both consider seeing each other quietly for a while without forcing the fact on his children's attention?
After a while they may be prepared/ able to regard your relationship with their father as a good thing, or at the very least as none of their business.