We've gone every other year for the last 8-years. We're heading back down in November. My kids are now 14 and 12 respectively. They love Carribean Beach Resort, so we've always stayed there. Definitely plan ahead. My wife has planning the trip down to a science. She constantly checks for deals on flights, when the meal plans are free, new rides, rides closed for restoration, new restaurants, etc. We map everything out 6 months in advance. We decide everything from where we want to eat to what rides and shows are absolute musts on each trip. Others have given great advice, and you really can't go wrong with whatever you choose.
My two cents.....we found that with young kids, splitting the day into morning and evening sessions works great. My kids were 4 and 6 years old on our first WDW trip. We would go to a park at open, head back to the hotel after lunch, relax at the pool or nap for 2 or 3-hours (believe me you'll all need the break), then head back to the park (or a different park with the optional Park Hopper) for dinner and stay for that evening's parade or fireworks show.
There's a lot of walking and waiting so a midday break is a must with a 5-year old. You'll be as exhausted as they are. There's nothing better then a midday nap, in an air conditioned room.
If your planning for an April or May trip, you should probably contact Disney now, to get things rolling. They will help you with everything and make the trip as stress-free as possible. We've never had a bad experience.
Best of luck and enjoy.....