I was there in early March. The butterfly exhibit was a let-down. There were a few butterflies clinging to the screens of the enclosure. There was only 1 butterfly flying around. It had been quite cold - maybe they didn't like the weather.
Weird thing about the butterfly exhibit - to keep the butterflies in, the exits have chains hanging down from the top. (If you remember the old strings of beads that people used to hang in open doorways like curtains, it's like that.) If you're walking in or out, you pull the chains out of your way. If you're pushing a stroller, it seems like it would be really difficult to keep the chains from slapping your kid in the face, because the child is in front of you. So be careful about that.
There were at least 2 little playground areas that aren't normally there. If you go to disneyworld.com you should be able to get information about the festival. I'd put in a link, but I just accidentally knocked myself off the website when I tried.
There are tons of topiaries, but I don't know if your little ones would be at all interested in seeing them.