The speaker of "Lullaby" knows that nothing lasts forever and that time passes. He includes several obvious symbols of time in his poem, and talks a whole lot about the fleetingness of life. But you know what? He seems pretty okay with the fact that nothing lasts forever. In fact, he thinks that the fleetingness of life makes it all the more valuable and beautiful. He's not lamenting time; he's celebrating it.
The speaker has a kind of carpe diem (seize the day) attitude in this poem. He says that since life is short, we have to make the best of our time here.
The speaker is actually much more pessimistic. He says that we've just got to accept what we've got, because we're all going to die one day.