Wow, on that link the OP posted, the editors choice for best bike is about as bad as kids bikes can be. It appears they did their clueless "research" for on line purchases without actually going to a quality bike shop. Clearly they don't know about good bikes. The title should have included "for cheapskake buyers".
I run a local bike coop where we get 500+ bikes every year including all those bikes in the review. Most are very heavy, more than half the weight of the intended riders. I challenge any adult to try to ride a bike that is half your weight or more. Those horrible bikes get tossed in our basement to get stripped for future recycling when we have the time. Most are cheap bsos with bearings that will wear out in 1 season, low grade cables that have so much drag that a child just can't operate the controls. Made of cheap steel. Unplated chains, derailleurs, freewheels that will rust solid if they get wet. Rode hard and put away wet is a quick end to the usefulness.
A good kids bike for actual dirt trail riding should be a name brand bike with an aluminum frame. Trek, Specialized, Gary Fisher, some Giants, etc. Generally name brand bikes are properly assembled and sold through dealers who don't sell on line. yeah, they will cost more but they work well and will hold up and can be passed from kid to kid or generation to generation. Bikes like a Trek 200 or 220 (24 inch mtb) is built to the same quality standards as their adult bikes.
20" bikes vary tremendously in design. Some are fall taller than others. Some are very low slung and will fit shorter children. One fit factor is crank arm length. Some of the better bikes have cranks with 2 pedal positions so they can be adjusted to fit the child. There are good bmx bikes (not sold at big box stores) but they are all 1-speed bikes with long crank arms and aren't suited for longer distance rides (bmx races are about 30 seconds long).
24" wheels are recommended for ages 10-14. However some 26" wheeled mountain bikes come in extra small frame sizes (12-13") and have about the same fit as 24" wheel bikes.. The advantage of larger wheels is they roll better on dirt trails.
Kids just don't need full suspension bikes. Even cheap shock forks add too much weight. Less weight with fatter tires will take up the bumps okay for kids on dirt trails.
Pound for pound, kids just aren't as strong as adults. I've seen to many getting off and pushing their bikes up hills. I've even seen that cant even do that with heavy bikes.
Frame sizes. Cheap bikes don't come in frame sizes. Good bikes sold at dealers do, mostly 26" or larger bikes come in a variety of sizes. Each manufacturer will decide what sizes to make and sell for each model. Some are labeled in inches, others just generic XS, S, M, L, XL. Taller frames are usually longer (seat to stem) and often have longer stems. Taller people aren't just taller in their legs, they also have longer torsos and longer arms so the bigger bikes need to be stretched out more. Every one is proportioned differently A decent local bike shop will work with customers to fit bikes by adjusting seat height, seat fore-aft, stem heights & lengths. Bike shops have a large inventory of stems and will swap them out to fit the rider often at no extra charge. Some brake levers are designed for small hands and can be adjusted for little fingers to reach.
