The other posters have given great feedback. You have to know that going with an 18 month old will be a "for the moment" trip for them. He/She will not remember anything so go in knowing that!
The youngest I took my daughter was 10 months. We did parades, shows, a few rides, danced to music at Club Buzz, and walked around experiencing the atmosphere at both parks.
Calif. Adventure has Playhouse Disney PERFECT for that age (all ages really), the block party bash which they will enjoy, bugs life (all rides for the real little ones) and bugs life has a water splash and play area...that is really fun! Monster's Inc. ride (very 18 month old friendly), the Animation Building is SO fun for this age with movie clips playing on large screens with music and a HUGE area to dance, watch and take it all in! Also in this building is Crush the animated interactive talking turtle (soooo cute).
Disneyland has trolley's and trains and such that run up and down main street that are fun, a parade, small world, dumbo, casey jr. train ride, carousel, petting zoo (I think it's still there?), toon town with Mickey and Minnie's houses (and others) that you can walk through and enjoy (really fun), Jungle Cruise, The boat that circles Tom Sayer's Island, Winnie the Pooh ride.
Is it worth it? Like others have said, it's a personal opinion that you'll have to make HOWEVER, to me...Heck Yah it's worth it! I enjoy being in the parks myself just for the atmosphere alone.
Just have to decide if it is for you guys
If you have any questions PLEASE feel free to ask away
