If you go to crocs.com, you can print off a sizing chart. My DD2 wears a size 5 shoe and I used the chart to determine that she needed a 4/5 crocs. Not every color comes in this size, only a few (I got the light pink). My friend who's dd is 1 and has a size 3 foot and a size 4 foot got the croclings in 4/5 (with a stretchy back strap) and the croc does not stay on her small foot, it falls off all the time.
My DD adores her crocs and wears them every day. She can put them on herself but when she gets mad she kicks her feet and kicks them off very easily.
I personally think Robeez are the best shoes for a 1 year old.