In general, DL does provide the same 25% discount for child fares for TATL travel in J, but you MUST phone to book the tickets as it cannot be done on the website. There is no option for passenger type Child. I have done this as recently as this spring - only reason I didn't book with KL (which you CAN do on their site) is because I had residual DL credits to use.

The already mentioned suggestion of cancelling your current booking for a credit is likely your only option to reclaim the lost value.