Our swim team does Donut Fridays but for an active (and already skinny) kid, that's hardly going to move the needle. OP, for swim meet mornings, my 10 year old likes and egg and cheese sandwich and juice on the way to morning meets, and I usually pack her fruit (cut up melon, apple slices, a banana) and simple carbs (Belvitas, granola, a bag of dry cereal, sports drink) for snacking during the meet. After, we do a meal with protein. Before evening meets, we'll do tortellini, a sandwich, or chicken nuggets to eat between warm-ups and events, and same snacks.