I don't think this passage is about the psychological concept of our "inner child," but that does not mean that Jesus is not speaking to the inner child.
The quality Jesus is calling us to is not necessarily innocence, but trust. The way children hold their parent's or teachers' hands when crossing a street, the way they believe what they are told. They way they accept that their livelihood does not depend on their greatness or even worthiness.
If we can trust God in that same way, the kingdom of heaven is ours.
Actually, my explanation here overlaps with Andrews, because in the Greek New Testament, the word we often translate as "faith" (see Andrew's answer) is sometimes translated as trust.
But trust is scarier for us, isn't it. I mean, for me. For now, though, I take comfort in that I don't need to be innocent, because I'm not. And I wasn't as a child either. I'm not talking about my abuse here, just about the rest of life.
Childlike smiles and laughter? A noble goal, and a nice picture of heaven.
"This is not my shame, this is their shame." Mona Eltahawy