When we took DD for the first time (at 5 months) she pretty much rode everything without a height restriction. MK obviously had the most rides for her but the other parks also had plenty to do. There won't be much your older children will want to ride that the little one can't.
Trying to remember off the top of my head, we did...
At MK: All Fantasy Land rides, Buzz, Pirates, Haunted Mansion (you'll likely have a harder time getting the 2 & 6 year olds on that one!)
At Epcot: Nemo (she loved it), Journey into Imagination, the boat ride in Mexico was still under rehab, I'd recommend that too
At AK: Safari, tried for the Nemo show (technical difficulties)
At DHS: Great Movie Ride, Little Mermaid show
She also really enjoyed just walking around looking at all the people, buildings, animals at AK, etc. We had our reclining stroller from home so she often took naps while we pushed her, giving the adults opportunities to ride "bigger" attractions.
Have a great trip!