Let's see...
PROS:
Dirt cheap compared to games like Call of Duty and Halo 4, plus no subscription payment thing required.
MANY mods for it as well. One example is Tekkit, though I'd recommend you use the Technic Launcher for that since it will install everything needed to get them to work properly. Gives you access to 4 different mod type setups with a click, Technic and Tekkit being the LARGEST of them.
Texture packs can breath new life into it once you get bored with the default textures.
MANY adventure maps that can be challenging and quite fun.
You can build whatever ye imagine, albeit in cube form.

CONS:
The only real con is that it WILL get boring after a while.
In short, get your kid MC.
