For husband, babysitters, and grandmothers you have to have ways to keep your allergy kid's stuff away from the other kid's food. So at my house, since my allergy kid is Landon, all his food, drinks, are on the left. And all his cups are green--for go. His brother's cups are all red--for stop.
I know it seems silly and a little simple but keeping it simple has kept mistakes from happening. And besides, it's the only way I can remember it.
Our little guy (he's 3.5) is allergic to peanuts, tree nuts and eggs. We are a nut free household. My daughter (she's 2) hasn't ever had a nut or anything made from a nut as far as I know. Perhaps a grandparent snuck something in, but I doubt it. We buy food that is "kid friendly" and food for adults. Kid friendly food doesn't have eggs in it, while my husband and I occasionally have treats with eggs in them. Since eggs and nuts are reasonably easy to weed out of a diet, I suspect we'll continue this way. It's not a huge deal since our son's egg allergy is mild anyway. Our families have been very good about supporting us and most of our family events are also nut free.
I don't know what we'd do if we had something that is more difficult to avoid, like milk, but I suspect we'd do the same thing. It's a lot of work to cook separate meals for different family members.
That's what I did! Lexie's is on the Left. They both started with L, so it was easy to remember. I like your red and green cup system - that's great! Working with daycares, babysitters, birthday parties, school, restaurants - that was never easy. People seemed so eager to try to include her on treats and things, but they had trouble getting it right. For example, she was given a peanut butter sandwich instead of a grilled cheese at the babysitters when she was 3 yrs old. The sitter used the same knife to cut both sandwiches. Reaction. Another time, she was fed tuna noodle casserole for lunch, the sitter was avoiding chicken. But, the casserole had cream of (something) soup mixed in: Dairy. We went to a restaurant and I asked "is it cooked in butter or margarine?" the waiter stumbled and said "butter?". It's hard for people to get it right, and sometimes they just don't understand and want to tell you what they think you want to hear. Or they don't know what all is dairy or what has chicken in it (like most turkey products are injected with chicken fat). I always found it easier to eat at home and bring allergy free treats to share in order to include my daughter and keep her safe.