The "I" Factor
Publishers automatically think newbie when they see a manuscript in first-person, but sometimes it works. Eccentric characters - as in real life - often live for good or bad in the "I" mode.
And, middle-grade readers prefer it, over third person, because it allows them to experience the character up close.
Now, the best-seller "Wonder" is written in multiple first-person and no doubt it was a risk for a young audience, but it worked.
So, never say never - even to first-person, okay?