That is very hard to say. Personally, I have never been to Universal/IOA. We strickly do WDW while we are there. I would give your children a choice on what to do. We have a 16 year old and a 20 year old. We have given them the choice before and they have wanted to just do Disney. So, ask your chicldren and see what they want to do while you are down there. I would suggest at least one full day of Magic Kingdom and one full day EPCOT. Then depending how much you enjoy MGM and Animal Kingdom, either do them in one day or two days.
If you plan on going into the parks all 8 days you are there, I would suggest doing 4 or 5 days in WDW and 3 to 4 Days at Universal/IOA.
But whatever you do, make sure you have a lot of fun. Memories will last a lifetime.