We go for a heavy carb dinner the night before then wholemeal toast with nut butter and a yoghurt drink for breakfast. She can't eat much if it's a really early start without feeling sick, but if it is a later start then I'll try to get a much bigger breakfast into her such as eggs and beans on toast.
For during meets, the skill is in maintaining a delicate balance of never full and never hungry IMO. I tend to send sushi, as it mixes carbs and protein and is easy to not go nuts with and have a snaccident (ie can follow an easy direction of "have 2 or 3 pieces straight after each race"), plus has the added bonus of being one of her favourite foods. Something like a pot of pasta would disappear in one sitting and it wouldn't be until she felt full and bloated that she'd remembered she needed to ration it out! Bitesize chunks of chicken or cheese are good for this too, and wraps or sandwiches cut into much smaller than usual pieces could work as well. I pack a small amount of crudités and a pot of fruit, a milkshake and/or a carton of fruit juice for if she feels she's flagging, and a token amount of sweets plus the sushi and another bite sized protein. She likes the protein yoghurts in the squeezy pouches too. Our senior coaches are against copious sweets at anything other than our 'for fun' internal galas, and warn against creating sugar highs and crashes, so to much disgruntlement, DD gets less sweets than she'd like. Sadly but understandably, nuts aren't allowed, otherwise I'd send a good amount of those (she tends to have some in the car on the way home though!).
Lots of people bring flapjacks and homemade cakes (particularly banana bread), but DD doesn't like to eat these during a meet for reasons she can't articulate, so they are out for us. Ditto bananas as a whole fruit.
I have tried sending her with salty sugary squash to rehydrate with, but she only wants water, so that's now what she has. I figure she gets plenty of salt from the food and sugar from the other drinks and odd sweet, so haven't pushed it. There was an episode of Sliced Bread on Radio 4 recently about sports drinks and the conclusion was the best choice was 1/3 fill the bottle with their favourite fruit juice, 2/3 water, then add salt until you can just start to taste it (you need carbs, sugar, water and salt in the drink).
I tend to only give a sessions' worth at a time so everything is eaten in balance, refreshing her supply in the breaks.
At the breaks I usually give a smallish portion of marinated fish with 1/2 a pouch of mixed grains (the supermarket version of Merchant Gourmet. I have the other half) plus some salad with a dressing. I try to pick a different style of flavour each time to make it a bit more interesting eg Mediterranean, Pesto, Mexican etc. There never seems to be enough time to properly digest it ahead of the next warm-up, so I am wary of giving her too much. Our Counties pool is just across the road from a Greggs, so she did have a steak bake one day this year when her gap to the next race was bigger... She never complains she's hungry whilst we're there, but will always need a big dinner once home!
The jelly probably wouldn't do much these days, as I think it is all now very low sugar and full of sweeteners? If someone has found a brand that still makes the jelly of my childhood, I'd love to know please for pudding (rather than swimming) purposes! Neither me nor DD1 like the taste of sweeteners, so it has been reduced from occasional treat to happy memory.