The natural imagery in "Spring," as well as the religious concerns surrounding the Garden of Eden, are centered on the idea of innocence. The loss of innocence – both in man's expulsion from Eden, and also currently, in children around the world – could be called the main fear or source of tension in the poem. Through the renewal of spring and its religious associations with the resurrection of Christ, there is also perhaps a hint of salvation from this seemingly inevitable loss.
Since all of our speaker's representations of innocence – spring, lambs, children – are temporary states (seasons change, lambs and children grow into adults), it is clear that, despite his prayer, our speaker knows that the loss of innocence is inevitable.