Actually, I think it would be better to teach them English and Na'vi at the same time. It would be like showing them a ball and saying "Ball, rum." and showing them a dog and saying "Dog, nantang". Ever since I started learning Na'vi, I thought "what harm would it do? after all, they'd speak both Na'vi and English."
Then again, I'd get calls from the teacher: "Ma'am, your son is calling the other children ayvrrtep again."
"Oh, don't worry about it, just say plltxe ni'Inglisi rutxe and he'll stop."
But teaching them only Na'vi? That would be cruel, because they wouldn't be able to speak with other kids at preschool, and their social skills would lag, and they would be outcasts until they learned English. If i felt the need to speak with my child in Na'vi, they'd definitely be learning English too.
Then again, it would be cool because then we could have private mommy-child conversations in fluent Na'vi. And when Mommy yells in Na'vi, you'd better shut up and fly straight!!