3/4 Size
I think children under 10 should probably play 3/4-sized guitars. As far as acoustic vs electric, there are good arguments for both…
Electric Guitar
For kids, I think electric guitars are easier to hold and play, and they also have a “cool factor,” which is motivating! Accessories: a small amplifier, an instrument cable, a guitar strap, a clip-on electronic tuner, and some picks.
Acoustic Guitar
On the other hand, acoustic guitars are great for kids also. The strings are a little harder to press down, but they don’t require an amplifier, which makes practicing and traveling simpler. Accessories: a clip-on electronic tuner and some picks.
Conclusion
Electric guitar is cool and easier to hold and play. Also, it rocks.
Acoustic guitar is a little harder to play, but has a richer sound. It doesn’t require an amplifier, so you can be more spontaneous.
Get whichever guitar you want! If your child sticks with it for a year, get them a second guitar of the other kind, and they’ll have all their musical options covered.