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The height barrier is a big thing. If your 4 year old is tall enough then id say its worth doing if you can budget for it as your 16 year old will probably love the days at Universal. We were never asked to take our toddlers shoes off when he was very close to the height restriction on a few occasions. As far as the 17 month old, youll probably be limited to the ET show and the kids area (providing Universal havent started demolishing it for the Nintendo Land by then) and the Hogwarts Express in Universal and Seuss Landing in IOA. I'd probably say around 16 years old is the perfect age for Universal if theyre into rides and particularly Harry Potter.

We didnt do Sea World or Busch Gardens as we felt the cost was too high for all parks and for what Sea World give you in shows, Disney more than make up for it elsewhere. The Rollercoasters would also be out of the question for your little ones. Busch Gardens is also very coaster heavy so it was an obvious miss for us. The way we looked at it with our toddler was we didnt try to cram in things that maybe werent quite right for their age or height or whatever and would do those things in future trips in years to come.

We tried Gatorland for the first time this year and it was quite a fun 2 or 3 hour trip. I probably wouldnt pay the entrance fee they ask but you can often find ticket companies throw them in as part of deals and if thats the case its well worth going for a quieter day walking around the swamps and enclosures.

EDIT: Volcano Bay - Really cool water park, but again youll probably spend most of your time in the tots area so maybe not worth too much extra expense if its not already part of any package you get. May as well just go to whichever Disney water park is open or Aquatica if thats included.