Keep also in mind that different places sell their feeders under different names. Big Cheese doesn't have mice weans, they have mice hoppers 7-12g and adult mice 13 -18g. Their rat fuzzies are 10 - 19g and the rat pups are 20 - 34g. Layne Labs classes their rat pups in the 20 - 29g range and fuzzies as 14-19g. When buying food, it's a lot easier to get the weight of your snake then go by the 10-15% guideline and weigh your feeders too, or try to eyeball feeding prey the same thickness (or slightly more or less, advice here varies) as the thickest part of your snake. There's common terms but not a whole lot of standard with the feeder labeling conventions- weans, hoppers, crawlers, pups... it gets confusing! Go by numbers or what you can directly observe and there will be a lot less mystery.
For your kid, going by the 10-15% rule 15-23g feeders would be a good number to aim at though slightly larger would likely not hurt them. Smaller rat pups, or larger rat fuzzies would be perfect. If you get a pack of f/t rat pups it will have rats through that whole range which will grow with your snake. What I did when I ordered my f/t stuff from Big Cheese was take them all out of the bag they shipped in and divided them up into smaller freezer bags by 10 gram increments. Two packs of rat pups lasted my girls perfectly up to the point they were ready for weans, and then I did the same from the weans up to smalls, and from that point I stopped weighing and just started eyeballing for general size. They get one small adult rat a week now and don't usually bother with weighing the food except a couple when I get a new bag so I have an idea of the average in it.