Firstly, book the part whenever suits your family. It's up to others to govern then own beliefs.
Secondly, not really sure what the big deal with tea and coffee is, so they don't drink it? Is tea and coffee such a massive part of life that that beings up a massive issue? Confused there that it's a big thing.
So ... Mormon here, firstly, tea and coffee thing isn't about caffeine. In fact we don't really know why, we mostly see it as an obedience thing, but tea and coffee is what we are specifically told not to drink (along with alcohol) but whatever the reason is doesn't affect our ability to drink hot chocolate weirdly, or eat it, in fact chocolate is my drug of choice 😂 generally we all live it a bit differently, it's all classed as the "word of wisdom" and there are things in it about eating meat sparingly and getting enough sleep, that some ignore and others take to the extreme. But drinking tea and coffee will be frowned upon.
The same applies with parties on a Sunday .. some family's do and seem don't. We don't, we try and keep Sunday's about service, and thinking about family, christ and various other church related topics and activities. We don't shop or do activities as such on a Sunday. Saying that, there are plenty who do, so I just depends on what their idea of "keeping the sabbath" means. If my kids birthday falls on a Sunday we do a party on another day but may have a "tea party" at home that day, but there wouldn't a party as such (although Frequently we invite others over to eat leftovers form the Previous nights party).
Although, you do have my sympathy, most places are geared up for Sunday parties so anything on a Saturday is a pain to find.