Call up the pizza place, ask for a manager, and say the following.

"Hi, I came in earlier and got a pizza as a walk in. When me and my son got home and went to eat the pizza crust was very burnt and hard to eat and we ended up throwing it out, is there a number for corporate you can give me so I can call and complain?"

And that's it!!!! The manager will ask you to let him replace the pizza rather than you call corporate and complain. Managers do not want corporate getting complaints about their store. This tricks the manager into offering a replacement rather than you asking for one. Take note, SEing is much easier by getting the target to offer the replacement rather than asking for it. Asking for a replacement just seems like your trying to get free shit.

Also make sure you say you walked in and ordered the pizza. You did not call ahead and it was not delivery, so your name/email/address wouldn't be in there system and they can't look your order up. IF they ask for a receipt when you come to get the replacement, say "no problem I'm sending my son to get the pizza and I'll send the receipt with him". When you get there to get the pizza say yeah my dad just called and sent me here blah blah. This is good for two reasons. 1, you can play dumb because if they ask any questions you have no idea your dad just sent you. 
2, if they actually ask for the receipt, say your dad dident give you one, they will replace anyways.