Children find a more upright sitting posture, slightly inclined forward, as a city or touring bike usually better than a racing or MTB posture. With a sporty MTB posture, some children suffer from their wrists. That is unfortunate, because then they leave the bike quickly. You can also not easily look around with a sporty bike position and overview improves stability and safety. Just stand on one leg and look at something at eye level and then look at the floor just in front of your feet. You will then experience that you are not in such a good balance with your eyes turned to the ground. If you get back problems on the bike, the saddle is probably not high enough, so
the knee must be slightly bent when the pedal is at the lowest point.
The knee must 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 that is unnecessarily heavy: If you get up deep from the squats, this is heavier in the beginning and the easier your right foot comes up. You must not bend or strain the knee unnecessarily. That could also be a cause of knee complaints. Lees ook: https://stip-kinderfietsen.nl/hoe-bepaal-juiste-cranklengte-kinderfiets/ https://stip-kinderfietsen.nl/hoe-bepaal-je-de-juiste-zadelhoogte-van-een-kinderfiets/ https://www.fietsersbond.nl/de-fiets/kooptips/welke-fietshouding/