Here's more info from the resident Rugrats expert.
When going towards the Universal Store on your left at entry, on the ground, you should see some big pointers indicating where Nick studios is. Between the H-B ride and right across the store that sells Nick merchandise, you will get to see the Rugrats! I'm not exactly certain when they will show up, but your best bet is during the early afternoon. I'm also uncertain if they will be out if they should be doing Slime-Time live. I have seen some of the characters out during the live show on T.V. so I don't know if they will effect meeting them during the time they tape.
To add more on this. The following Rugrats characters that you should see are Tommy, and my persoanl favorites Angelica and Chuckie. I wish they had Kimi or Phil and Lil out
Shame on you Universal/Nick!
"That guy who likes Minnie Mouse.*"
*Wanna know why? <A HREF="http://wdwinfo.infopop.net/OpenTopic/page?q=Y&a=tpc&s=40009993&f=55009194&m=493099902" TARGET=_blank>click here for my story!