As far as I can tell (doing this pretty quick)... there are 6 ways to arrange 3 numbers for a product of 36. This is as follows:
WE DON'T KNOW THE NUMBER OF HER HOUSE... so these combinations, when added give:
The reason he needs more information is because the house number is 13, and there are TWO combinations that result in 13. Therefore the final clue indicates that there is a definitive eldest child, meaning that the combination includes two 2 year olds and one 9 year old child (learning to play piano).