We also did IOA only, back in 2004 when the E's were 4&5 and they LOVED it. Both parks actually have quite a lot for little ones. The crowds for WWOHP are/were a bit mind boggling but by the time you go it won't be the peak summer season or brand new so even if you do IOA and the WWOHP section, I can't imagine it will be as bad. I am more than happy to answer any questions about it!
We saw more characters in US (Scooby and the gang, Woody Woodpecker, the Simspons, more but I'm spacing on who) and all the Nick Jr stuff is there like Jimmy Nuetron etc. Plus Shrek in 3d...which is always fun. Some great shows on that side, the Beetlejuice graveyard review was a highlight of the total trip for me, and sweet rides like ET.
On the IOA side you have the Seuss area, a really fun classic comics area (Popeye, etc) and lots of super hero's(spidey, the justice league, etc...saw all of them "live", plus of course Harry Potter. Which was AMAZING. Ana may be a bit young for it though all of you would enjoy the theming and the detail even if you haven't seen a movie or read a book.
The FJ is an AMAZING AMAZING ride.
On both sides, if you are into the thrill rides for yourself with child swap, well, lets just say the parks rock. IOA in particular in our opinion.