i usually say the same thing, do IOA first for the big rides.
if you are not into coasters, the jurassic park area has the dinosauer area, seussland for dr seuss rides and exploring play area.......extremely colorful and all themeing follows his books, and the super heroes come out in marval land at different times of the day on atvs
you can do pictures with the heroes at various times at the shop near spiderman. spidey is a fun ride but don't know if the 4 year old would enjoy it as much as the 7 year olds would. it has some spining to it and can be loud.
the studios has a play area for all kids, ET ride is fun, barney show, characters in the park at different places, jimmy neutron ride, shrek, MIB which is a similar set up at buzz lightyear where you ride in a car and shoot aliens to gain points.
this link will give you the height requirement for some of the rides.
http://www.universalorlando.com/pi_height.html
universal has the child swap program for kids unable to go on a ride.