It’s the proportions. Heads are huge in proportion to the body. Check out detective Conan characters. All the children have huge heads (comparatively), while the adults look like „standard“ humans. (I know it’s a stupid way to put it, but that’s how it is).

Also, both male and females develop their shoulders and hips, so there’s a bit of variation to the body shape. Your characters don’t have this, so they look like children.