J.H.
I am a BIG croc's fan and wear them, almost exclusively. They are very easy to care for (the dishwasher is the ideal place to wash them!), comfortable, and distinctive! I wear them year-round, and in the winter wear the ones w/o holes and put on heavy socks.
I would suggest either thin socks or powder and frequent cleaning. Also, they suggest using "croc butter" on the shoes to keep them in good condition--look at the croc website for more info. You might be able to ask customer service a question about the allergic reaction you child is experiencing via their website as well.
Did you know there are croc outlet stores at Lake Ozark and Branson! They are fantastic and inexpensive there, as compared to the prices you pay at regular retail stores in the city-sometimes as low as 4.99 to 9.99 per pair for some of the "wilder" colors!
Hope things go better for you little one with his crocs--believe me, I understand why he loves them! J.