Using AnimationTree
Overactive I hope I wrote everything correctly and understandably
I use a translator. So I might misunderstand something.
They should both have the same rig and mesh
The model has one mesh. The children's mesh is a modification of the adult mesh. (There is also an intermediate form around 9 years old.) And there are no problems in growing up a static model. But the rig is different. Different proportions: arms, legs, head. And problems start when trying to animate a growing character.
Therefore, the problem is in the assignment of the correct rig.
I suggested to the MakeHuman team to move it to the Godot platform, but did not meet with much enthusiasm.
There is a solution, of course, but it's extremely expensive. It's to make a bio-mechanical model of a human being. Starting with the skeleton and covering it with muscles, fat, and finally skin. It's a huge job. But then any parameter can be controlled and everything will look as natural as possible.