Did you just find his will or his will and probate records? I hate to send you on a goose chase but I think the kids names will be in his probate records unless he left everything to his wife and son.
I'd look for Margaret's marriage license. Both sets of parents might be on it.
If not, then I'd look for the newspaper announcement of their marriage.
Look for her newspaper death notice. Her parents might be in it too. Successful Searching!