I recently came across the following article on Yahoo on crocs which can be dangerous for young children. They are comfortable, but I wouldn't have my 2 to 5 year olds wear them till their foot size won't be...
FITTING TIPS GUIDELINES General Birkenstock Fitting Guidelines Place your heel within the heelcup. Your foot should fit the inside edges of the shoe with enough extra length (up to 1/4" - 1cm) for movement...