In most aspects of Halachah a Kattan is out of the realm of the Mitzvos.
As for the Minhag of saying Shalom Aleichem during Kiddush Levanah I was wondering whether there is any preference to say it to an adult. Since a Kattan doesn't "do" or "accomplish" ritual factors it would be akin to greeting a lamppost or being a human whom we care for he is no different from any adult. Or, perhaps it would depend on which reason given by SimchasTorah we'd go by.
Being that there is no great or serious ramifications one way or the other, I'd be happy with whatever can be brought to the table, be it anecdotes, Svaros, insults, sources etc.