I specialize in teaching young kids--ages 3-10--and I can't emphasize how important it is to find a guitar that fits the body of the child. Just keeping the darn thing on their lap is a trial, not to mention the huge feat of actually fretting a string. Why make it impossible by giving them an instrument that doesn't fit them? I see intonation problems as minor inconveniences in comparison to issues of playability. Sure, that out-of-tune note may be grating on us teachers, but it's not going to damage the child. Your primary goal is not to make beautiful music right away--it's to give them a sense of accomplishment and confidence, and to have fun.
About finding a teacher for your six-year-old--it's hard. I think I'm the only guitar teacher in Seattle who works with kids that young.
I wrote an article about
teaching young kids guitar on my blog--lemme know what you think!
Rob