Today's reprinted Peanuts says:
It's possible that Schulz got this from an actual textbook, instead of making it up. I decided to see if it was possible to deduce the answer. Not by direct calculation; I wouldn't know where to start. But by brute force. Easy enough with Excel; you just enter a sequence of possible ages for the daughter in one column, and then calculate all the derivatives in other columns.
And the answer? The man is 41. His daughter is 7 (7x6=42, 1 year older than he is now) and his son is 10 (7+3). That has to be it, because 10 years from now, they will be 17 & 20 (= 37, which +14=51, 10 years more than his present age), and nothing else fits. Note that "the combined ages of his children" means their ages then, ten years from now, and not their ages today, because then the man would have to be 11 and his daughter 2 (2+5=7, 7+14=21).