Odd one...
An award reservation viewed from the account of the person flying vs. the person whose miles were used shows that extra miles were purchased. For example:

1 Reward 77,500 Miles
Additional Taxes/Fees $481.10
77,500 Miles and $166.10
Purchase 8,892 additional miles
$315.00
Total Price 77,500 Miles and $481.10

The same reservation viewed from the account where the miles came from correctly shows only the tax of $166.10.

There was no additional charge that I can see - so it looks like an odd display issue.

I see this with two different itineraries - different children flying, same source account.