OKay I think it depends a lot on your child. Is your DS timid, does he like Disney movies - what movie/scene is his limit for 'scary' is dark an issue, water, noise, smell? it depends on the kid.
It also depends on how long you're going to be there and if you'll be going back to the first park - or if the first day is it for the park you're in.
At EPCOT our girls LOVE the Land, a tame boat ride, looking at veggies and fish. Nemo was too much for our almost 4 yr old last year, but the 2 yr old LOVED it. This year our 5 yr old will love NEmo - but last year Bruce in the movie scared her so she wasn't going on a ride where she might see him.
Spaceship earth is fun - but I think for the older kids b/c the younger ones don't 'get it' the fire scene there bothered our older DD - and thus our younger DD didn't like it either.
At MK the Carousel, dumbo, it's a small world, winnie the pooh are all good first rides - peter pan is dark, snow white can be scary with the dark and the witch.
Buzz Lightyear is a fav of our girls, as are the tea cups - we haven't taken them on Haunted Mansion, but this year they've requested it.
Before going we watch youtube videos of the rides so the scary ones might not be an issue. But it depends on the child.
This trip we'll go to HS first - figuring there's only a few rides there, but shows that we know they'll love. A gentle introduction with lots of time for breaks. Day 2 is AK again slower, early night - a few more easy rides and lots to look at - then we'll have a much better idea of what they can handle at MK since our 5 yr old wants to try all the 'big' rides this year. But I won't bring her on BTMRR or SM if she didn't like Star Tours (yes I know the fundamental ride is dif, but I'd rather wait to introduce the ride so the first experience is positive).
So I personally think EPCOT is a fine place to start. Oh and Mexico has a nice ride, but I'd start with the Land, Figment, the Seas, then Mexico for rides, but don't forget there's so much more than rides to entertain the kids - our girls really have a blast at inventions, and the little splash zones. They also enjoy the concerts, character meet & greets, and of course shopping (I"ve taught them well
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