Some thoughts:
- Enjoy! DD7 started taking guitar lessons last year, and I started teaching myself as a result. I'm enjoying it more than she is (and she loves it).
- At 18.5mos, I don't think there's a wrong way to do it, except maybe going overboard and teaching too much too quickly. Letters on the keys and letters on the sheet music sound like a solid way to start to me. As she progresses, showing her how the lines on the staff correspond to the notes would be a good next step.
I can't comment on method beyond that, having never learned the instrument properly myself.
- I think the piano/keyboard is a superior first instrument for a child that age, because the physical demands are much lower. Holding a violin properly is somewhat awkward and strains the neck. And if the strings are anything like they are on even a Spanish guitar (with its softer, nylon strings), they'll cause discomfort for sensitive fingers.
Plus, there's a lot to be said for how instantly accessible and rewarding a keyboard can be. Anyone anywhere can sit down and randomly select white keys to make music that is non-horrible. That's not the case with a stringed instrument, where you have to know the right fingerings to stay within any particular key.
