Suggestions for changing food sensitivies?

I am so exhausted (ADHD parent of AuDhd, SPD, PDA, OCD kiddo - age 8). Every time I don't have the exact perfect food or we run out of 1 ingredient it causes such turmoil. My daughter has very specific safe/preferred foods. Generally when she's in a groove we're fine for a few months, and then her taste buds seem to change and we're in turmoil for weeks until we can find a new thing.

This week it's water. She hated drinking water when she was younger, but recently she's loved water. Then for some reason last week it started to taste different to her. I swear she can taste different minerals.... Tap, ice cubes... she won't drink it now. She said she can taste things floating in it and it tastes horrible. She woke up last night with leg cramps from being dehydrated. I bought a new Brita filter to see if that would be better.

How do I help her expand what she will eat and drink? It's so enormously exhausting all the time. And one of our younger ones seems to be following the same path. He's 4, and his list of safe foods keeps getting smaller and smaller.