Right now, in non-peak/holiday season, the dining plan for a child is the same price as QSDP ($11.99). During peak season, the kids' dining plan jumps to $12.99.
However, you can't only upgrade the child's plan, all plans in the room must be the same. For adults, this means that during normal season, you would pay $11 per night per person extra, and during peak it would be $13.