When I was a kid, we called them "cooties". I looked under that name in the DT and found a song called "Army Bugs", sung to the melody of "Sweet Bye & Bye". There's only one verse given, so you might want to come up with others. But that creative process could make the whole thing more interesting to the kids involved - just think, a chance to write your very own "cooties song". I recall a Japanese expression for the wee critters, that included both varieties. Please forgive the spelling, but I think it was "chisai tomodachi", or "little friends".