I was a competitive swimmer for a number of years and it is the fondest part of my childhood memories. I ended up quitting when we moved to a new state and I had trouble settling into a different program. I wish now my parents would have encouraged me to stick it out a bit longer. I was just having trouble with the transition of everything else. So, I actually don't even have a problem with the idea of encouraging a child to stick with an activity if they really have big dreams about it... at least for a temporary period -- we all get discouraged here and there.... but as her parent, listen and watch... you'd know when it has become too much.
I'd just work your dd's schedule into your lives as best as you can and then talk about and watch for any signs of stress or fatigue. Who's to say you can't pull back if it gets to be too much? Better to go for it now!
Another point though, is that I believe that while we're young, we have the best access to learning a whole range of sports and activities. While I would never profess to be a pro in any of these areas, I did participate on teams and take lessons in piano, drama, figure skating, basketball, badminton, track & field, voice, photography and the list just goes on. Now, as an adult, it's harder to find time to participate and learn something new. Plus, I think as kids, we are able to adapt and learn more quickly. So, I would also still try to let your dd dabble in other activities as long as it doesn't over stress her AND with the understanding there is much less pressure on her performance... and that these are just things that are fun to learn as you grow on your journey in life.
Good luck to your dd!
Oh as for snacks...Do you need something she can fix at home quick or carry with her and eat at school/on the way to the gym?
how about bagels or english muffins w/ peanut butter or cream cheese? Celery w/ pb or cream cheese, string cheese, or the pre-cut shaped cheese -- comes in star and moon shapes and more -- on cold days -- instant oatmeal, snack packs of cottage cheese (sprinkle cinnamon sugar on it), yogurt, or even gogurt, snack packs of peaches, natural applesauce, hard boiled eggs, whole grain pasta pre-cooked and refrigerated can have some light butter or oil and parmesan cheese added to it and be microwaved pretty quickly. You can pre-make some customized trailmix including almost any dry ingredient you'd like from cheerios, chex mix, peanuts, raisins, choc chips, marshmallows, pretzels -- throw it in a bowl w/a cover and shake up to mix. Also, you probably already give her Gatorade or equivalent type of sports drink but just thought I'd mention it.