I think this song is about Robin struggling with aging. He wishes he was a kid again, however, he wants to be a kid that didn’t have all of the flaws/problems that he has. He was born like that, he can’t help it.
He feels insecure about his body and his feelings, he wishes he was a kid when things like that went over his head.
The chorus I think is about how he never fit in with the “normal kids.” when he says he’s hanging out with the bullets in his mouth I think it means he’s trying to conform to a normalcy for example with other kids. But when these kids are gone he can be himself and hang out with animals and stuff like that.
I think it’s also about growing up with depression and anxiety (I don’t know exactly what conditions, but). It can be really easy to want to go back to your childhood as a young adult and wish for a time when you were still figuring things out and you could mess up more, but he knows that his childhood was still a scary place because he didn’t know how to deal with his mental health. So, it’s wishing for a time when he didn’t have to have the responsibility he has now, but knowing that his childhood had it’s own disasters too. If you need clarification, let me know.