I have a 5yr old little madam myself. And I must say that you are very brave in undertaking this with two of them plus an 11 month old baby.
But I promise you that at the end of the day it is going to be an experience you will never ever forget. My trip last year with my daughter up in Punda and Pafuri was one of my best.
You could easily take a round trip from Phalaborwa gate to Orpen gate and/or vice versa in the two day planned day trips.
The elephant museum in Letaba is a must stop for the 5yr olds.
As a general rule I never try to drive for too far and too long with kids in the car. They can get extremely bored and busy in a car. I always make sure that there is enough room inside the car to make them move around freely. Plenty of pillows and padding in the car is always of great help.
Stopping at one of the camps or picnic spots for a picnic with the little ones is a winner. Olifants and Satara have pleasant day visitors areas. Make them help you pack a nice picnic blanket and basket and let them help you set it all up once you are there. The anticipation of the picnic is a nice negotiable carrot to hold in front of their noses once they start getting busy in the car.
I also make sure that there are enough toys, colouring books/crayons and maybe even a kiddies cd/tape in the car to keep them busy when sightings are scarce. Some folks also have invented awesome games with regards to sightings. Also read this topic
for a whole lot more of info with regards to going to Kruger with kids.
Remember sunscreen and Satara has a day visitors pool if you guys wanna go have a dip.
If they are interested in birds then I suggest that you get either one of these books. The Kruger one probably being the best suited for your trip.
Birds of the Kruger for kids[/url]
My first book of Southern African birds
Most important piece of advice I can give you is: Enjoy every moment of it!!