And sleep as I in childhood sweetly slept, Untroubling, and untroubled where I lie, The grass below—above the vaulted sky.
The speaker continues, using a simile (comparing with "as") to tell us that while he lives with his creator he wants to sleep, just like he did in childhood. If he sleeps like that, he won't bother anybody ("untroubling"), and nobody will bother him ("untroubled").
The grass will be below, and the "vaulted sky" will be above. Well that's a very charming image now isn't it?
By the way, "vaulted" just means arched. If you look up at the sky a certain way, it sort of looks like a vaulted ceiling. Supposedly.
All this business about sleeping and lying makes us think of the speaker's actual grave, rather than Heaven. For example, a lot of headstones say things like "Here lies Bill Shmoop" and "Rest in Peace" (rest=sleep).
This doesn't necessarily mean the speaker is actually talking about going to the grave, instead of Heaven. His body will go to the grave and rest, while his spirit or soul will go to Heaven to "abide" with his creator. Maybe.
The rhyme scheme in this final stanza is the same as in the second stanza, by the way: ABABCC
Even in this state, though, the speaker still has his writing chops. We also get some more terrific examples of alliteration, what with all the S sounds and the "Un-" words. (Alliteration of the S sound is specifically known as sibilance, just in case you wanted to know.)