Even if you manage to get a child to stand still long enough to take a shot, catching their eyes looking directly at your lens can be next to impossible. The Look Lock from Tether Tools holds your iPhone directly above the front element of your lens, so you can have them focus on whatever colorful character they're currently infatuated with while it appears that they're looking into the lens.
Again, this isn't something you should break out unless it becomes the only viable option. Once the iPhone is part of the equation, it's going to be hard to take it out of the picture without a tantrum. So, get what you can naturally first, and then bust it out if you need it.