Steph,
Yah, I also think it's not good to bluff and say you'll buy next time and never honour it. Celeste loves swimming and every single day, she'll wanna go swimming. There was once we told her we will bring her to swim on a saturday and she acutally remembered all the way. When saturday came and WE FORGOT about what we said, she whined and whined and reminded us to bring her to swim. :s Powerful memory.
Now, we tell her we will go swimming on weekends only when the weather is good. Hehehe. Must add the condition.
Anyway, my whole family is quite cooperative when we come to eating junk food. Try our very very best not to eat junk food (potato chips, biscuits, anything to cause constipation) infront of Celeste. We hide, we eat when she's not around and do everything to avoid letting her eat. If she sees us eat, then we quickly distract her with something else but that doesn't really work acutally. haha.. anyway, i think you need to send the message across to your in laws that why you think all those food is junk to Z. like it's processed, too salty, too sweet, not good for Z, will cause him to be ill in the long term etc. Ask them if they want Z to be ill????