My daughter was almost five last year when we went.
My biggest piece of advice is to pay close attention to the little one. The parks are just so huge, with soooo much going on , that they tire a lot faster than they normally would. We took a break every day, and spent quiet time in the room, and a little pool time. It really made a difference in how everyone was able to enjoy themselves.

Also, if taking a break doesn't work well with your plans, try to schedule a sit down lunch rather than a grab and go. If you have a leisurely lunch , everyone can cool down and relax for a bit, which is better than nothing.

another trick of mine was to buy disney toys and glow items before we left home. I raided the disney stores during sales, bought tshirts and those flashing light toys for dollars a piece. I also came across party packs of glow necklaces, bracelets and earrings at a discount store. I got entire packages for $2, where the necklaces alone at WDW are 2-3 times that. It's a huge meltdown saver when the kids want something and you can whip it out of your bag!

Enjoy your trip!