So I calmed down and decided to address this after all. This is the note I just sent to my MIL. I held it together and was as polite as possible (I hope) and slanted toward their (in-laws') happiness:

(MIL), I have no idea why Colin's reaction was so overblown with you guys today as he has had slip-ups before (didn't realize chicken nuggets had wheat and had one from his friend's plate at school, for example) and he did not freak out the way he did today.

I think it possibly may be related to your own stress about it as you said you were "scared" and I am not sure what that meant...I don't punish, I don't hit...so I really am not fully understanding this unless it's just general stress and I think he felt your degree of upset.

I do not want you and (FIL) to feel so stressed when you're with the children. Grandparent time is supposed to be fun and relaxing for both the grands and the kids. So I propose we change things up and the next time you want to visit with them, perhaps rather than it being a full day at your house, you all can see a movie or go to the park or something.

If you would like it to be us rather than you reinforcing gluten-free and so on, one of us can go with you. It would be fun and it would leave you and (FIL) to simply having fun with the children rather than this degree of stress.

I think shorter periods (an hour or two) plus myself and/or (DH) being there could help. I'm not sure what else to do, so am giving this as a suggestion.

(GlennaGirl)