Just back from a trip with a five-year-old boy. Something that I may have overlooked in my planning (oops) were the play areas for younger kids - he wanted to play at every one he saw! Top three were: the inside play area before riding Dumbo (definitely plan time to let him explore there even if you don't have to wait for the ride - it's really well done! my guy must have spent 20 minutes playing at least, I think he liked that more then riding Dumbo), the play area as you exit Splash Mountain (this is kind of hidden and off to the side but he saw it and took off, and there were lots of other kids his age to play with), and Casey's JR Splash and Soak (go prepared with a spare set of clothes or trunks to change into before letting him loose in there! Really nice bathrooms right behind that area

). He also wanted to play in the Bone Yard in Dinoland at AK but we were caught in a downpour and missed this one.
When my oldest was younger he also liked the Honey I Shrunk the Kids play area at DHS but that one made me nervous because I thought it was difficult to keep an eye on him in there!
Also my 5 y/o loves meeting characters. He'd met the stuffed characters numerous times on two previous trips (Mickey, Pooh, etc) but this time we met Tinker Bell and had such a great time with her - he couldn't stop talking about how nice she was, how sparkly (body glitter lol) - it was such a different experience for him meeting a face character.
As for rides, at MK I'd definitely plan on:
Astro Orbiter
Tomorrowland Speedway - FP (boring line for LOs!)
People Mover
Buzz
Dumbo
Peter Pan - FP
Pooh
Ariel
Teacups
Magic Carpets
...but I'd suggest you try out 7DMT (w/FP!) or the Barnstormer before BTMR. I didn't think of it because everyone else in our family loves BTMR but it was too dark and too loud right at the start for our 5-year-old. He also really hated Haunted Mansion. Splash Mountain mine liked to watch but didn't want to ride himself (thank goodness for the playground there!).