Given that it is another Ramazzotti's song title, in this context it has a definite meaning. Otherwise it is a rather original expression. You know, poetic licence allows songwriters to use rare idioms or even to invent new ones.
Literally it is "child in time". The word tempo=time means "during all his life" here. I think it alludes to an adult who, for some reasons, didn't grow up completely :) Time passed, but he remained a child in some respects. E.g. one that still hopes and dreams as a child usually does. In common parlance you would rather say: "un eterno bambino", "un bambino non cresciuto", "un eterno ragazzo" (an eternal lad), "un ragazzo mai cresciuto" (a lad never grown up), "un ragazzo mai diventato adulto" etc, but these sentences, although clearer in meaning, are of course less poetic than "un bambino nel tempo".