One should always exercise extreme caution in treating a poem as an exercise in autobiography. However, in the case of Rilke in “Childhood” it's a perfectly valid approach given that autobiographical elements often crop up in his poetry, and that his childhood was notoriously unhappy.
Childhood is presented to us by Rilke as not a lot of fun. As he tells us, it was a period of his life when nothing ever happened to him except that which happens to things and creatures. The reduction of a child to the status of a mere thing or creature is an indication perhaps that the speaker was ill-treated as a child. At the very least, we can reasonably conclude that it wasn't a happy, carefree childhood.
More than anything else, the speaker was “lonely as a shepherd” when he was a child, overburdened by the vast distances that separated him from adulthood. Childhood is supposed to be a preparation for adulthood, yet in the case of the speaker, he simply moves from one lonely, meaningless state to another.
And the speaker's childhood most certainly was meaningless, itself a damning indictment on his upbringing. Sometimes he's reminded of his childhood by the rain, but even then he cannot say what it means.