In case the CM won't give you only one piece:
How many pieces does the kids meal come with? That might be a better option. Or find someone to split with. DH once asked for an extra biscuit, but didn't know about the extra fee. The nice person behind him offered to give him her biscuit because she didn't want it. The CM was fine with that, so everyone was happy. Win Win. If you don't want to keep the extra piece for yourself, maybe someone else in line would want it?