I think you are doing a great job helping your children make good food choices. There are plenty of food choices that will work for a swimmer before a swim meet or a swim practice, and as you have found, what is good for one swimmer is not as good for another. Some of the choices will vary based on timing - what works if eaten three or more hours prior to a meet might be a bad choice eaten 30-minutes prior to a meet!
The pre-swim meal decision requires some real-world testing, and with a day or two to go before the meet it may be a bit too late to try new things. I suggest you go with what you know works for each of them, regardless of whether it is a breakfast, lunch, or dinner type of meal. What is their favorite pre-swim meal? Go with it! It could be pasta, noodles, rice, cereal, toast, eggs, a sub-type sandwich, pancakes, fruit, waffles, or a peanut butter sandwich - it does not make a huge difference, as long as it is a meal that hits the main food groups, is easy for them to digest, and is familiar to them.
Get that main pre-swim meet or workout meal done two to three hours prior to swimming, then "keep the fuel tank topped off" with easy to digest, lighter foods - fruit (apples, oranges, bananas, raisins, pears, etc.), power bars, sport drink, pretzels, pop-tarts, a simple sandwich (peanut putter and banana, banana and honey, jam, etc.), low-fat pudding, rice cakes, plain toast, etc.
What if you are traveling and not home? What nutritious choices are there on restaurant menus? No kitchen to make your children their favorite pre-meet meal? You can always grocery shop and look for good equivalents. Instant oatmeal only needs hot water. Some fast food restaurants have choices that might work, too - as would a regular restaurant. Talk with the restaurant staff, review the menu with them and maybe they can make something off of that menu in such way that it meets the needs of your swimmers - it cannot hurt to ask! I have had no problem asking for white-only eggs, no butter on things, steamed vegetables, etc. when I explain what is going on and ask politely.
Feed 'em what you know is good for them and what they think of as tasting good and that they feel good eating. Later on, try a few different things prior to swim practices and learn what other choices may work for your swimmers.