When we went at Christmas (the first day A&E moved from DL to DCA) they had an A&E fast pass kiosk set up w/ cast members at the end of the hollywood street (don't recall the name) just past Animation Academy where the actual meet & greet is. I found you could get a fastpass even in the afternoon but if DD6 really wants to see them I'd just go get the FP as soon as you arrive at the park so you can check that off

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No one knows how the Olaf meet and greet will work starting in May other than it will be somewhere.
We didn't get to do the sing-a-long at DCA and I've heard it should still be running in June - I'm really looking forward to seeing it next month when we go.
There is a Frozen play over in DL at Fantasy Faire which we also haven't seen yet.
Not sure about what the nighttime and parade stuff will be post May 22 but there was a frozen "pre-parade" at DL and I am pretty sure there was some Frozen stuff added to the DCA WOC show too (although I find the night time stuff hard to do with DD5 as she goes to bed early).
DCA has a little Frozen store which is set up like the shop up in the mountain just in front of the sing-a-long theater. Small but cute.
I also read there are some Frozen themed treats which may be around into May and June so we'll be keeping our eyes open for them (a cupcake and ice cream sundae).
So both parks have Frozen stuff but if you only could do one park you'd have to do DCA in order to meet A&E.
Oh - DTD has a Frozen themed store too. We have not brought DD5 there yet as we tend to not buy much "stuff" - we choose to spend our money on experiences instead
