We are actually going to be doing the same thing while at DisneyLand and found that the only tour that our 8 year old can go on is the Warner Brothers Studio Tour. They have a VIP and a Deluxe package. The VIP is about 2-3 hours long and the Deluxe is about 5 hours long and includes lunch. My kids are also into Harry Potter and they have a large area dedicated to this, so it works out well for my 8 year old.
The Studio tour at Universal is open to everyone, but is is part of the attractions at Universal Studios Hollywood and not a true standalone studio tour like Warner Brothers.
Is the deluxe tour at WB worth the extra? We've been wanting to do a studio tour for awhile now. I've lived here almost all my life and have never done one.