Cannulas for a two year old?

We've capitulated and have decided to add O2 to our Collection Of Stuff
(it's tough to file IFR over Northeastern California and the Great Basin
without it) and are trying to figure out what to do when it comes to our
two year old. He's nuts about flying and generally accepting of wearing
his headset, but we're skeptical that he'd wear a mask for any length of
time.

Does anyone know of a vendor of child-sized cannula or mask? Do any of
you folks have any experience flying on O2 with kids in this age range?