The first year we went to WDW, the kids were 5 and 8. Our son (the 5-year-old) was tall for his age, so we rode EVERYTHING! I saw lots of disappointed kids, though, that were too short for rides, and had to wait for other family members. We also saw a family with (and I am not making this up) a 5, 4, 3, and 2-year-old - and Dad was carrying a newborn in a sling! His comment to my husband as we passed him on a long line at the MK was, "This isn't a vacation - it's torture!" (Mom looked very stressed.)
I guess the point is that if you want to take a toddler or pre-schooler, just plan ahead. Be aware that there are rides your little one isn't going to be allowed on, and don't go near them. (Let the older kids go on ahead, if you're comfortable with that.) Take breaks, carry water bottles, carry prepackaged snack foods or juice boxes, premoistened wipes, sunscreen, extra socks...whatever you think will keep everybody reasonable comfortable. When the youngest (or oldest!) member of your party starts to fade, LEAVE and hit the pool!