But this is elite sport & missing out is part & parcel of being chosen as a future GB olympian
I am familiar with this turn of phrase, and I think I have some experience of the type of elite training the OP is talking about (it's not being 'chosen as a future Olympian either, although the hope is that a few of the kids at the Elite clubs will go the distance).
At 8? of course its fine to go to a party. At 10/11 the expectations change, ime, but at this age, she needs to more than just train, and some of it should be fun and parties with her friends.
I would also resist the temptation to talk about 'her Olympic training' and her being 'an elite athlete' (when she's an 8 year old at an Elite club) etc etc etc. You're proud of her, obviously, but I've seen several parents do this kind of thing and it's a bit clunky.
The training gets way more brutal intensive - so let her do stuff now. If she's 100% on board with the program when she's 10, then she'll be making the choice to miss things herself in order to stay in the Elite team. If she doesn't want to, then open the door to just being a keen hobby gymnast instead.