I could not work out how clue 2 helped at all. You don't know the house number the agent calls at or next door...
a+ b + c = n is all very good, but only confuses the issue by adding a 4th unknown. So I ignored it.
There are 3 possible answers, but assuming the 3 children are all different ages & being told they are all whole numbers, it leaves 1 possible answer.
I won't spoil it for anyone else but would like to know if I'm correct & if all 3 clues were used.