I would suspect Dumbo would still be functioning in May. I would almost expect at least some downtime for it while they move it, but i've heard indications that the move wouldn't happen until sometime in 2012. At the very least, I'd expect them to not shutter the existing dumbo until at the very least they've completed the ToonTown demolition and built the new foundations for Dumbo.
Barnstormer is currently closed since it was located in ToonTown. It will return however when the new circus land opens.
As for other little kid rides closed, Obviously Toontown is closed, so that includes the play area there. Pooh's Playful Spot was also closed last year to make room for the expansion. Pooh's Tree however has been moved over in front of the Pooh ride as part of it's new interactive queue.
Beyond that, I don't expect any further closures as part of the Fantasyland Expansion project between now and May. The only other stuff that may be closed would be the usual shortterm refurb closures you can find listed in various places.