The first thing to be aware of is that the rules are different depending on your age.
Specifically for those 13-years-old or younger, any sort of “costume” — the sort of thing one might wear for Halloween — is allowed.
In addition, proper footwear is always required when walking around Disneyland, so any costume that resulted in bare feet would not be allowed.
The only official restrictions would be related to costumes containing material that will drag on the ground, as the powers that be don’t want anybody getting tripped up.