Dreamworks always had them. I think for Disney it really started when they had Robin Williams as Genie.
According to some documentary I think children's movie really started using celebrities after the success of Dr Dolittle. Apparently the first cut just had a bunch of no names and bombed at test screenings, so they recast the voice actors with celebrities and huge success.
this seems like a thread made for manabyte.
It didn't start with Genie. The poster above you is correct. Disney was casting "big" actors as soon as they could get them.
You need to understand that even into the 60s, animation wasn't looked at as "serious" enough for a huge star to do the voice in it. It wasn't until the Renaissance that it really kicked off, but Disney was already starting to cast big name stars in the 80's prior to Little Mermaid. Hell for an 80's animated movie, they had Billy Joel as one of the lead characters with Bette Midler in Oliver & Company, and the same year they put Kathleen Turner in Who Framed Roger Rabbit...back when she still sounded female.
After Beauty & the Beast was nominated for BEST PICTURE people were knocking down Disney's door to do voices in their movies. The Genie was an exception because Good Morning Vietnam made Robin Williams a metric fuckton of money and he did it for $75,000. But that backfired after he sued Disney, and won, due to Katzenberg and Eisner completely tossing out their agreement with him. Basically Williams agreed to do it for scale pay in exchange for the character and his name not being used to market the movie. What did Eisner do? Slap a giant Genie on the poster. Fucking idiot.
But at the same time half the animators were doing Aladdin, the other half was doing Lion King and they had an all star cast there.
The difference is that Disney/Pixar doesn't sell their movies based on the voice actors. Dreamworks does. You'll never see a big name star's name over the title of a Disney/Pixar animated movie; but you see it on EVERY FUCKING Dreamworks piece of shit.