Nintendo DSs and Gameboys have their own movie-playing functions, but are limited to certain cartridges that you have to buy. If you're willing to figure it out for yourself as well, a Sony PSP is a better media player.
Then again, for an 8-year-old, a Game Boy (or a DS) is a better bet, since those play games more suitable to your kid's age. Also, if you buy some equipment, your DS can be modified to play various media files.