Are you going during Frozen Summer Fun?
Are you staying on site? (if so .. DHS would just be easier)
How many Disney Park days are you doing? (if so .. adding a day to go to DHS would be relatively inexpensive)
But .. that's a tough age to do to DHS. Especially with an 18mo old who you think may not sit through shows.
I am going in August with a 2,3, and 4 year old and this is what we are doing:
Disney Junior Show
Toy Story
Meet Characters (in various places like
Pixar Place)
Go to Disney Junior character meal at Hollywood & Vine
See the "Frozen Summer Fun" stuff (like the parade)
The 4 year old (girl) will go to the Ice Palace Boutique.
See the Frozen Sing-along show
See the Muppets show
Beauty and the Beast show
Eat at Pizza Planet
We MAY go on the Great Movie Ride, but we are worried bout the Alien scene being too scary for the little kids. So .. we may just have adults go.
From there .. it may be rough .. I am going to take my son (3 year old) .. to Indiana Jones and Lights, Motor Action .. but I am not sure if the 2 year old will be interested or if the 4 year old would be either. None of the kids may enjoy the loud explosions. The adults will probably take turns going on Star Tours and maybe Rock n Roller Coaster or Tower of Terror later.
I think there will be plenty for us to do there for a full day.
If the kids can make it that late.. we'll see Fantasmic or Frozen Fireworks (or both)
All that being said .. I would do DHS over SeaWorld .. but I went to SeaWorld once and just wasn't impressed enough to want to go back any time soon.