Lily (10) only wears swim shirts with swim skirts. I always get them from Costco but they don't have sizes larger than T8. She is skinny and can still fit in size 5, but the shirts are too short(she likes to keep it tucked in the skirt while swimming). She also likes her hip fully covered. I had to make this for her (way more expensive and more work than buying) as there's nothing on the market for her now.
I made my own pattern for the top and used a Jalie pattern for the bottom part, but didn't use the A line skirt in the pattern. Instead, I made it a half circle skirt.
Lily has been very picky since she was a baby, complaining about socks and undies not comfortable enough. Lia (8), on the other hand, can wear all styles, including this style, one piece, tankini and bikini, although I almost never buy bikini, as that requires a lot of sunscreen.
I remember going to all the shops trying to find jeans for a size 0 or 1 for my oldest daughter. When I did find them I had to pay high. She had outgrown the children size but was too small for adult size.
Lily is extremely skinny (she still wears 5 year old size for undies) and it's hard to find pants that fit her. I have to alter the waist or make her pants.