As it is usual for SW to offer 2nd day free I think you can easily make use of both days ( although you probably won't need to be there from openning to closing) . With little ones ( that need a lot of lifting up to see things) it's better to take your time as there is a lot of walking. SW has a nice kids area with bouncy castles, climbing frames and a water area ( bring swim suits) that will keep them occupied for a good hour or two if needed.
Although I think IOA is a WONDERFUL park, it doesn't really offer much to kids of that age. Universal has a few more things and is probably a better bet out of those two