Found it -- the title is Red Planet, first published 1949. The Wikipedia article notes that the native Martians here are physically and culturally very similar to the ones that would appear 12 years later in the much more adult-oriented Stranger in a Strange Land. (Apparently, SiaSL was originally proposed by Heinlein's wife in the late '40s as a kids-oriented "Jungle Book on Mars", so Red Planet may have been an early experiment in this direction.)