Children should definitely be using shinguards that come up to only a few inches below the knee. They should be equipped with plastic, not padded, ankle guards. Also make sure they have velcro straps to secure the shin guards in place.
Older soccer players typically go for the smaller shinguards, which do let you become more mobile, but more susceptible to injuries. Also more players prefer to secure their shin guards with medical tape at the top and bottom of the shin guard, instead of the traditional velcro scraps. You may also want to consider a Nike shinguards sleeve instead. You can pull these over your shinguards and secure them in place. These are definitely more comfortable than velcro straps or tape.
Although your probably looking for Lotto products, some other good brands of soccer shin guards to keep in mind include Itech, Nike, Uhlsport, and Adidas!
On a safety note, you should never wear shin guards that are too tight, too lose, or cracked. That is just telling you it's time to upgrade to better soccer equipment!
Sports Authority -
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OnlineSports -
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Soccer.com -
Original site for all sorts of soccer equipment, shoes, cleats, and shin guards.