Barleans Omega Swirl for kids. Its a creamy, sweet lemon flavor. My kids would drink it straight out of the bottle if I left it out! I have to buy it at the health food store as it is a refrigerated item. They make one for adults too:)
A friend of mine swears by Miralax for her son, and you can mix it in anything. Apparently it is not habit forming. Ask your ped if youre not familiar with it!
Sorry for the digestive issues. That must be very difficult.