purchased a size small for 16 month old (shoe size 4/5), size medium for a 3.5 yr old (shoe size 8/9). couldn't even get these on their poor little feet! i LOVE this idea, just wish i was able to use them. i will return them to the store.
As a previous reviewer, these got holes in the bottom after one use! Have used them 2 more times and one sock has a hole 1.5" in diameter. They're useless now. LOVE the concept, they just aren't durable enough for regular pool swimming!
Bought S for 2 year old, M for 4 year old. Took these to indoor water park where rubber soled swim shoes not allowed because hazardous on slides (stick to walls). My kids loved them but soles had holes after one day of wear! Very very disappointed. The idea is awesome but not worth money for one wear.