This is tricky...
If you're going to buy your kid any instrument, and price is a issue, it might be better to start them on something that is already cheaper by it's very form.
A Yamaha recorder for instance can instill the fundamentals of music but is in of itself a very respectable instrument that can be purchased for around 25-30 dollars depending on the size.
A 20 dollar guitar isn't going to be able to cut it. It isn't a respectable instrument. It will likely do more damage than good, even to very young players who just want to make noise, as it is literally impossible to derive any musical knowledge or progress from one or through playing one and will therefore eventually bore and frustrate.
Guitars have a lot of key elements that work together and as such need to be good quality. The machine heads won't stay in tune or turn smoothly enough to tune easily. It will very likely warp and have intonation issues right out of the box. Fret buzz on poorly dressed or warped necks will screw you up enough to mess with knowing if you're fingering a chord shape correctly or not.
You can buy a perfectly acceptable second hand smaller size classical acoustic for near to 100 dollars or less. My recommendation would be to not bother with this.