Unfortunately you cannot use conditions to hide/show sub-questions (array rows).
You could:
1) Use JavaScript to show the appropriate number of array row as the dropdown is changed.
2) Use separate questions for each child and hide/show them with conditions. You could apply a little CSS to make the separate questions appear to be one.
Cheers,
Tony Partner Solutions, code and workarounds presented in these forums are given without any warranty, implied or otherwise.