Don't assume that a "left-handed" child will play left-handed. I'm left-handed, and I play guitar the same way as anybody else. I picked up my instrument the first time, and it felt right that way.

We call people left-handed if they write with the left hand. However, very few people who write left-handed use their left hand to do everything.

According to books I have read, people usually don't know ahead of time what hand they will use to do a task (except for a few tasks, such as writing), and they may not do the same task with the same hand upon repeating it.

So keep an open mind about how the child should play.