Answer:
The ages are 6, 6 and 1.
to solve, you want to write down all the 3 numbers whose product is 36.
1, 1, 36
1, 3, 12
1, 4, 9
1, 2, 18
1, 6, 6
2, 2, 9
2, 3, 6
3, 3, 4
Here's the hardest part, the fact that the doorbell clue was not enough to solve the puzzle means that if we add up each of these options, we get at least two results that are the same, and we need more information to decide which one.
These are
13: 1, 6, 6
13: 2, 2, 9
Then to find the last hint, knowing that there IS a youngest child, means the smallest child doesn't have another
sibling in the same age, meaning that 2,2,9 doesn't work, and we are left with 1, 6 and 6.