I have done baskets like this for fundraiser auctions and I second what Elena B. posts below. If possible do a "younger kid' basket and an "older kid" one. If you only do one basket, I'd advise making it a younger-kid art supply collection only.
I think that parents bidding on baskets at these events are not really thinking in terms of "I want to do art too" especially if they have younger kids; they might see things like real oil pastels or expensive, serious paintbrushes as seeming out of place in a basket that also contains crayons, fingerpaints, etc. -- hence the idea of baskets geared either toward younger or older artists but not trying to cover both at once.
The adults bidding on kids' baskets are really focusing on what's in there for their kids, and won't give that much attention to whether there are art items in there that interest them. If you put in nice quality brushes, for instance, that a teen or adult would use, you would also need to put in paints of the quality to go with those brushes--expensive! And not useable by younger kids, really. (Yes, of course they can use the good stuff too, but it is pricey and not the same as art supplies that will wash out!)
Getting a local art museum to donate a membership, or a special tour if a membership is too pricey for them to donate, is a great idea for ANY age but be sure the donation covers a family membership (so parents and chiild are all covered) and/or a tour donation covers both parent and child.
Is your fundraiser doing any baskets geared directly at adults only? We had huge success with baskets on themes such as "parents' night in" (DVDs, fancy foodstuffs that kids wouldn't care for, a gift certificate to a wine store because in many places you cannot legally put wine into an auction like this one) or a basket with a gift or discount certificate for a spa plus spa items for adults, etc. See if a local photo studio will donate a gift certificate for one professional family photo -- that also got a TON of bids the year we had it. Get a local performing arts place to donate tickets and put them into a basket including whatever is related --picnic gear if it's an outdoor performance venue, for instance.
But mixing up things grown-ups like and can use with kids' items might not be the best use of the money you're spending to fill the baskets.