Children find a more upright sitting posture that leans slightly forward on a wooden balance bike, such as a city or touring bike, generally better than a running or mountain biking posture. With a sporty posture, some children suffer from wrists. It's a shame, because then they leave the bike quickly. You can also not easily look around with a sporty bike stance and the overview improves stability and safety. Just stand on one leg and stare at something at eye level, then stare at the ground right in front of your feet. You will then experience that you are not in such a good balance with your eyes on the ground. If you have back problems on the bike, the saddle is probably not high enough. The knee should be slightly bent when the pedal is at the lowest point.
The knee should be slightly bent when the pedal is at the lowest point. and the other knee should not be higher than the hip joint. If the knee rises higher than the hip joint, that knee bends too much and is unnecessarily heavy: if you rise deep from squats, it will be heavier at first and the easier your right will lift. You should not bend or straighten the knee unnecessarily. It could also be a cause of knee pain.