The scale does tilt lower on the side of 1st person POV in MG fiction, but the 3rd person side is by no means empty. The reason for the imbalance is readers themselves: tweens tend to focus inward as they really struggle with who they are for the first time. It suits their mindset to be inside a character’s head, experiencing the story for themselves. Third person POV risks making them feel a step removed from emotions and events. A benefit of choosing 3rd person is that you get to describe things outside the character. Don’t base your POV choice on the tilt of a scale. Write one of your scenes in each POV, then ask yourself which reveals more about your protagonist’s personality. Your comfort writing the POV matters, but not as much as the character revelations.