Although I haven't eaten at PM since it became a buffet, the last time we did eat there, the CMs were very flexible regarding "child" credits vs. "adult" credits. My oldest was just under 9, so still a child on the Disney
DDP, but didn't really want a kid's meal. Our server heard him saying he didn't want a kid's meal and she said, "let him get whatever he wants. There's no difference in the credits." We asked if she was sure and she said yes, they do it all the time. So we let him get an adult meal and when my husband went to check out, the cashier didn't say a word about using 2 A and 2 C, even though we had 3 adult meals and only 1 child. This was for dinner, using the Disney DDP.
Things can always change, of course, but I do know the CS credits on the dining plans are not broken down by adult/child - they're pooled, so they're not keeping track of how many of each you order. The only time you may have an issue is when you're ordering at a CS and the CM sees that you have 2A2C and only allows you to order 2 adult meals and 2 kids' meals. And apparently, the CMs at PM don't care.