The problem some of these posters are having is that there is only one adult and one child on their key card, not two adults, so they can't just go through the line if the CMs are paying attention to what's on the key card, like they are supposed to be doing. I'm not sure you will get that for a pizza as if you want a pizza meal for you and your child, there isn't a child's menu alternative to sharing a large pizza. I don't think that counts as breaking Disney's rule about the child credits but you could get a cashier who doesn't understand that. If you were ordering individual meals, then yes, you'd need to use the child's credit for the child's menu. Ask a cashier before you order the pizza, if you are worried it won't work - then if the cashier says you can do it, go to that cashier to pay.
Same thing for the (no longer offered) Picnic in the Park, there was no children's menu alternative for that meal.