Unless a particular person or object leaps to mind, I would be inclined to tackle this task backwards by first examining the subjects of Blake's poems and the points he derives from them. Both the Songs of Innocence and the Songs of Experience are full of children, animals, and plants.
Songs of Innocence contains a great many young children, particularly boys: the chimney sweeper, the little black boy, the little boy who is lost and found, and the baby in "Infant Joy." There are also the children playing in "The Echoing Green" and "Nurse's Song" and frequent images of the infant Jesus.
Songs of Experience also contains various images of childhood. It is worth stressing that the division between the two books is not absolute. Blake changed the location of some of the poems more than once. While it is generally true that the children in Songs of Experience are more bitter, it is certainly not true that they are more unfortunate. It is striking, for instance, that "The Chimney Sweeper" is placed in Songs of Innocence, while "The School Boy" is in Experience. Blake frequently makes the point that it is the attitude, not merely the situation, that determines innocence or experience.
Given this treatment in Blake's work, it might be interesting to take the same person, animal, or object and use him, her, or it as a representative of both innocence and experience at different times or under different circumstances. At what point, for instance, do you think a child transitions from innocence to experience? You might have the same child representing one and then the other a few weeks later. To try the same idea the opposite way around, experience turning to innocence (perhaps in an older person) would also open the question of whether innocence, once lost, can ever be regained. A somewhat more basic idea, but one which exposes common prejudices, is to take two animals or objects which are generally perceived in very different ways but that are, in fact, very similar; a squirrel might symbolize innocence and a rat, experience.