My sister's wedding is garb-optional, and she specifically has mentioned the wee ones dressed as her cadre of fairy sprites. I forget who proposed it first (my dad?), but a pooka/púca/pwca was suggested for the Spud. Which is neat, because his Nana has been calling him "a little pooka!" for some time.
The pooka is a shapeshifter, though. It's most typically in the form of a dark horse, all the better to give terrifying (but harmless) rides to the gullible. It's most famously in the form of a six-foot tall invisible white rabbit named Harvey.
My current thought is a bunny suit, maybe with red ears or, if that's too obscure, little teeny wings on the back.