Yes, you can go to both parks in one day.......... however, if you are not staying onsite, therefore not having fotl, it will probably be tough to do it "all"!! Depending on how many days you are there, I'd try to do both sides........... we had ap's for seaworld, and we enjoyed it, but once you go once.......... well, you've done it, you know what I mean?? Frankly, if your kids are coaster fans.......... I'd skip seaworld!!!!!!
I know your little one would LOVE universal........... there is soooo much for them there, and IOA does have a kiddie area, my son just preferred us........... he loves nickelodeon...... so he was familiar with those rides.
We also did slime time live......... and he met dave and jessica...... so he was sooo excited.
Anyway, just my 2 cents.......... but if you can I'd do us/ioa and skip seaworld......... although, it is great if you've never been, I guess since we've been a lot........ I don't get much out it anymore, and my boys LOVE us/ioa. ALSO
I'm not sure how long you'll be down there, or where you are staying, but if you can........ stay onsite at one of universals resorts.............. fotl is "fantastic" and you will be SOOOO glad you did it!!!!!!