Epcot is my kids favorite park - and has been the biggest hit on all four trips. The first trip's oldest child was barely 3. Second trip was 3.5 years old. Third trip had both a 3.5 and 6.5 year old. Fourth trip had a 2.5, 5.5, and 8.5 year old. In the first three trips, we only had one day at EP so we stuck to Future World. There is a lot for kids in the World Showcase as well, but due to limited time, we did just FW. This last trip, we did 2.5 days in Epcot and really needed more.
If you have a 40" child, think about Test Track and Soarin. Though my kids like Soarin, they absolutely love, love, love Test Track -even at 3 years old. We ride that at least 4 times every day we're at Epcot. My oldest loved Ellen on the last two trips, which surprised me since it's not really a little kid show. All three do well at it and no naps are taken in it - which was my intent with the 2 year old this last trip. I'd do Ellen if you have extra time or need some down time, but not work around your plans to squeeze it in. Also, Mission Space was loved by my 5 and 8 year old on this last trip. It's a 44" ride, so I'm not sure if that's a possibility for your older child. We stick to the green side, which is the more mild side. I'm very prone to motion sickness and had no problems on green side. (I skip Canada since it makes me feel sick).
My 2 year old adored Living with the Land and Spaceship Earth. Nemo actually scared him. The other two did fine on Nemo at 3, so it likely depends on the kid. I wouldn't start with Nemo just in case. They all enjoyed Captain EO - though it can be a bit scary during the first half of the movie for some young kids. Journey and Circle of life were fine for my 2 year old.
Don't forget about the Innovention buildings. My kids could spend an entire day in those two buildings alone.
On our second Epcot day, my husband booked over to Soarin' with the two older kids and everyone else at rope drop while I took my sweet time getting there with the 2 year old. That meant he had nearly 10 minutes chatting with Stitch since he was the only kid there. It also meant he got to meet the characters inside the Pavillion with no wait. By doing that, we weren't able to do rider swap with Soarin', but it was worth it to me. On our first day, we all went over Test Track immediately at rope drop and they let everyone skip the briefing which helped us ride it lots more times in a short amount of time.
Your 5 year old may enjoy the Kim Possible missions in World Showcase. My oldest loved them. My 5 year old wasn't into it, but many 5 year olds would be. Instead, she loved collecting stamps for her Duffy Bear cut-out. (Which they'll provide you with at any country). That let us do the countries as a group - with the younger two collecting their Duffy stamps and the older one doing the Missions. He did 7 missions over two days.
