IMNSHO, for what you describe...
...I'd spend a few minutes at WallyWorld and other big box stores and take a quick listen to those plastic three (or four) piece "portable" systems and go woth the one that meets your needs and sounds best. Make sure it plays MP3s if that's critical but I wouldn't go out of my way to make sure it plays cassettes. I don't think kids care about those anymore, if they even know what they are.
These may be big and ugly, but they can pump out a lot of sound for a small price.
I recommend these for lazy and/or technically challanged friends for outdoor use who don't want to bother with running speakers and wire nowadays. They tend to be cheaper and easier overall and can provide more than satisfying sound without all the complexity of wiring, zones, selectors, controls, etc....