I can really commiserate. I was in the position of telling one daughter that she was a dairy-allergic celiac, and also simply taking another daughter's diet and removing a crapload of favorite foods because she had to go SCD and since she is developmentally delayed was in no position to understand the sudden changes. I just really wanted to tell you this: it will be okay. And that you aren't taking stuff away - you are replacing it with something better. If you yourself truly believe this, it will sink in with the kids, and new food favorites and new food traditions will take the place of the old.
At this point, the celiac manages her diet exceedingly well and no longer feels sorry for herself. And even the special-needs one gets the connection between food and stinky farts and diarrhea and poopy pants.