Your only option is going to be cancel and rebook (and use the difference for a future fare).
Delta doesn't offer child fares and I don't know why they'd be obligated to provide you with one KLM offered. If you bought from DL on DL codeshare, they are free to use the fare rules they have in place, they aren't bound by the KLM fare rules or offers they make.
Any reason you didn't book this directly with KLM in the first place so you could directly apply the KLM 25% discount