"Ah. Clarification. It's not assumed in these games that you're supposed to roleplay being not trustworthy. Unless you've got a card from the older kids telling you to do that, but, at least at the age this game is usually played, they'd always tell you in advance if cards like that might be handed out."
"Though in this situation, if it's the last round, there's not much of a loss from carrying out your part? If you both witness the randomness generation and it says you get the lower side of the split, failing to follow through at that point just causes nobody, including you, to be the winner. The game instructions don't say that you do any worse by scoring lower than average - you either win or don't win. It's a bargain where you don't actually lose anything from following through, even if you lost, which is part of the reason I'd expect it to work even in a lower-trust situation."