I didn't have special WDW rules but we did have expectations for behavior that were generally the same as at home. One thing that I will stress is don't threaten if you aren't prepared to follow through! I can't tell you the number of times I heard "if you don't stop xx we are going home" well even kids know your probably not going home!
Watch some planning and or ride videos and find out what each child is interested in - we did a list of rides, attractions and shows for each kid in order of importance. Don't underestimate the resort pools and play grounds - kids that age need to move and play so we tried to find time every day for this.
Make sure everyone stayed hydrated, fed and understood - we stopped for snack breaks, bathroom breaks when necessary. We also made sure everyone knew that they would ride their list of must do's eventually. Then just have fun - ride the rides, see the shows and if a child is truly misbehaving give the warning (forfeit a ride, 10 minutes on a bench, no ice cream treat ... whatever works for your family) and follow through
Plan your meals, for us it was the larger main meal as a late lunch sit down as we all needed the break from the stimulus and the heat, breakfast was in the room and dinner was quick serve. Look at the menus and plan ahead as to what will work best for your family.
If your trip is longer there may be days when you just have to abandon plans, try to alternate late night shows (when they return) with early morning starts. and be willing to head back to the resort for a swim in the afternoon if things aren't going as planned.
Souvenir's work differently with every family but when the kids were young I had a good idea of what they would actually enjoy vs what looked fun at the moment so I would generally steer them to things I knew would be lasting fun for them. When they got a bit older we moved to a $ amount each day that they could carry over/save for the next and then eventually to a $ amount per trip.
Sorry this is long but if your still with me - but warn the 8yo that unless it's a 3 seater she won't be able to ride with the 5yo.
Same with the 5yo they must be seated with a "responsible party" over 14
5 and 8 are such great ages for the WDW parks - research rider swap for the rides the 5yo can't ride, realize your not going to see/do it all so just focus on each day.
Enjoy!