The answer is "A Child". This is because the author mentions feelings and smelling the moss on the trees/violets or flowers, as indicated by the line "Running over the club-moss burrs; I inhaled the violet's breath". The speaker also mentions seeing and hearing the rolling river again as indicated by the line "Again I saw, again I heard, The rolling river...". While the writer does mention a child he is using it as a metaphor as indicated by the line "Beauty is unripe childhood's cheat". He does not literally feel, smell or hear a child.