If you want to know what the official ball should be, go to FIFA.com. It will give all the material, circumfrence etc.
Other than that, it's simple; there are three sizes used in youth soccer to accomodate the ages of the players.
U6 and U8 size 3, the smallest ball.
U10 and U12 size 4.
Above that size 5, the size that is used in professional soccer.
Just remember not to put so much air in the ball that it feels like a canon ball or leave it so flat that it feels like a soft balloon when kicked.
If you can press a slight dent into the ball with moderate pressure, it is good to go. Remember that as it warms up the air (actually the moisture in the air) inside will expand and make the ball harder.
Balls are fairly inexpensive but can put a good dent in your wallet if you let it. You might buy a decent ball for ten to fifteen bucks (more if you can afford it at higher divisions) and buy practice balls for less. When a panel comes lose and starts to lift, get rid of it. Punch a hole in it so some kid won't pick it up to use. Imagine a ball accidentally hitting a player in the face. That hurts, but will do more damage if a sharp edge from a lifted panel hits that face. It's not worth the few bucks you think you'll save.
A good ball will say: "FIFA approved," or something to the effect of "Official FIFA size and material."