do some things but definitely not all of the entire park. If you decide to go for that amount of time, you can try to ideally see as many of the major attractions as you can and just save the rest for another time.
Also, depending on the crowds on that particular day (although more manageable than WDW) you might not be able to see as much as you like. So, I think if you and the kids would want to atleast get a taste of what Sea World is about, a half a day could be nice.
But, if you think personally that this amount of time may not be worth spending the money then you might want to put it on the to do next time list.
I have found that to fully enjoy all that Sea World has to offer that atleast a whole entire day and even 2 is preferrable so anything less especially to someone who hasn't experienced the park before can possibly feel like you're rushing through.
So, if you don't mind just seeing a couple of things this time around (saving others for another year) and taking a somewhat increased pace, then I think it would work out fine. I think it is basically up to you and whether or not you would really like to visit for a short day.
As far as small kids, I feel most of them love Sea World and it is a big hit with them. I have been to the Orlando park many times, and there has always been absolutely lots of small kids there at every turn as most of the shows such as Shamu, Clyde and Seamore, and the Dolphins are especially designed with a young audience in mind.
Whenever you decide to visit, I am sure your kids will have a blast; hope this helps.