Giant and Merida are both good. Giant is good-looking, but more expensive. I don’t recommend buying these brands. Although it is hard to ride a good bike, children have more energy, and inferior bikes can be more efficient. For good physical exercise, the most important thing is that the size of the bike should be suitable for children to ride, and then the child's riding posture (I won't go into details on this website, this is the key point).