Will a bike with 11" frame and 20" wheels be suitable for 120 cm high child? Is there general recommendation about what should I pay attention to, when choosing a bike for a kid?
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I don't know either of the bikes you mention, but the main thing to measure is the inside leg measurement, not the total height of the child as this is the most critical to get right for the bike fit.
One UK company which specialises in children's bikes is Isla Bikes. Their website has some very good measurement guides that ensures a good fit.
If your child has set their heart on one of the bikes you mention, I would recommend taking them down to the LBS to try them out for size before purchasing. There's no substitute for trying it out.
It's hard to answer that question, because people and bikes can be built in many different ways. Bike fit is a complex art, but here's the crude and simple version: have the rider straddle the bike's top tube, feet flat on the ground. If there isn't at least a couple inches of space between bike and crotch, then the bike is too big. On the other hand, you don't want it so small that your knees are hitting the handlebars while pedaling.
For a bike that's mainly ridden on pavement, you want the seat high enough so that the legs are almost fully extended at the bottom of the stroke. For off-road riding, or for kids that are still learning to ride, you want it lower.
The first thing to look at is "standover height". The child should be able to stand straddling the bike with both feet flat on the ground (and still comfortable clearance for "the valuables"). This is usually a bit easier on "mountain" style bikes, but don't take take too much advantage of that or you may get a bike that is too large overall. And if the bike really will be ridden off-road you want more clearance. But getting a bike that's grossly too small is not a good idea either.
Second, look at seat height. If this is a "starter" bike then it's best if it's possible to lower the seat enough that the child can sit on the seat and with his/her toes balance the bike and push it along. However, there needs to be enough adjustment range built in so that the seat can be lifted high enough to get (nearly) full leg extension at the bottom of the stroke (plus a little extra for growing room). (Start raising the seat a half-inch at a time once the child is steady on the bike.)
Finally "reach" -- generally if the height is right the reach will be OK, but check it, since some small bikes are actually built for larger kids. The child should be able to grasp the handlebars without having to lean forward more than a little -- we're not wanting to see an aero crouch here.
Note that for the first two measurements you can work from leg measurements, if you don't want the child shopping with you. (Use your best judgment regarding "clearance".) "Reach" will generally be OK if the bike is reasonably "normal" in form-factor.