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:
- 1x6x6
- 1x4x9
- 2x2x9
- 2x3x6
- 3x3x4
- 1x1x36

WE DON'T KNOW THE NUMBER OF HER HOUSE... so these combinations, when added give:

- 1+6+6=13
- 1+4+9=14
- 2+2+9=13
- 2+3+6=11
- 3+3+4=10
- 1+1+36=38

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).

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