The tar pits rock for little ones! The beaches...
http://la.mommypoppins.com/lakids/100-things-to-do-in-la-with-kids-before-they-grow-up

We went in May with our 5y and 12m old. We visit a children's museum in Pasadena by chance and my preschooler LOVED IT. To tell you the truth, I think it is better than Please touch museum in Philly!

The baby also had lots of fun.

http://www.tripadvisor.com/Attraction_Review-g32859-d634417-Reviews-Kidspace_Children_s_Museum-Pasadena_California.html

A place called PLAY in Santa Monica.

Peterson car museum

http://www.tarpits.org
La brea tar pits.

Just had a fabulous two weeks w my twin 3 1/2 year olds and family. Noah's ark at the skirball is the coolest! It's not religious at all--its a fabulous museum w animals made of reclaimed objects. Touch everything. Annenberg beach house was also great. Getty museum was a hit. La county museum (lacma) has a fantastic art room for kids. Love the science center downtown but go during the week to avoid the crowds. Kids also had a blast at the pony rides and train at Griffith park. Have fun!

Taking a trip out to LA in a few weeks and I'd love some advice on fun things that you've done with your 4 year old (will have a 18 mo old in tow as well). Not interested in Disney.
Ideas?