I would let them know as quickly as possible. Our gym has already started registering for meetings in the winter and you won't want to get stuck with registration fee's if you can help it. Our gym does have a policy for mandatory meets and valid reason's for missing. They also state in our contract we are still responsible for coaches fee's if we have to miss.
As your kids get older lots of sports have opportunities for away tournaments, meets, etc. Even school activities like band and choir travel for competitions. For the most part, we do not treat away gymnastics meets as vacations. The gym has rules about what they can and cannot do at away meets anyway, so they are pretty subdued. They actually state it is not a vacation and should not be treated that way. Usually, they are close enough that they only require 1 overnight stay which helps. My dd (level 7) does both gymnastics and soccer and my ds (non-gymnast) plays soccer. His teams do not traditionally travel, but, my daughter does for soccer and gymnastics. For us, fairness is allowing them both to participate in activities that they want and competing/playing where is required by the team. I figure in the course of their lives, they will both get to experience stuff that the other doesn't and that is perfectly fair. Normally only one of us will go to away event and the parent at home does something fun with my ds. We make sure that he doesn't miss out on his activities due to her having something out of town. If he doesn't have something we both go, and give our son the option to come with us or stay with grandma. Normally he chooses grandma as its alot more fun then watching a soccer game or gymnastics meet.
Lastly, the away things where you get to stay in a hotel, have team dinners and hang out with your teammates is really where the bonding happens. Last year my dd's team went to their first really far (plane ride needed) meet. The small team hung out the entire weekend and by the time they competed on Sunday were relaxed and hit their routines. From that point forward, every meet they improved as a group and by state, every one of them was in the top 5 of their age group. At the first meet of the year, only 1 placed in the top 5 all around.