I like those leather slippers. They keep their feet dry, as well as reducing the kicking issue.

I just traveled with mine in them at that age. They weren't doing much walking and everything's climate controlled. Shoes just weren't needed. Of course, once they started walking, and wanted to do so, they wore real shoes but up to then, Robeeze were the way to go! (I'll give that product a plug because I tried several brands and they were the best).