"Don't think he really took on board much of the meaning of Blake's poem beyond it having lots of exciting descriptive words. But he'll come back to it when he's older and get a richer understanding. Thing is, it's quite important for them to have a taste of great literature - if it has echoes of familiarity when they finally do it in depth I think that helps. Warms them up in some way."
I agree entirely. We do children a disservice if we assume they should be exposed only to topics they are likely to understand well. I think this one is a great choice. Let it wash over them, and they'll take from it what they will, which may be different for every child.
Many of the poems I most enjoy are ones I don't understand fully. That's part of what draws me to them. Poetry isn't all about being able to unpick what the poet meant in a definitive way.
I've told "Tyger" to my four year old several times recently. (She is not particularly verbal, rather the opposite.) She loves it. I explain a few phrases when she asks. Exactly what is going on in her mind while she is learning is often a mystery to me, but that doesn't change the fact she is learning a great deal.