I don't know if we'll make it there this summer, but if we do it will be a surprise for our kids. Not quite sure how we'll pull it off since they're all home for summer & packing without them knowing will be hard.
We did take them last year right before Thanksgiving & pulled off a great surprise. We picked them up from school (minimum day) and just started driving. After being in the car awhile they started to ask questions, so I told them we had to go pick up a Christmas present - which was technically the truth

As it started getting dark & we still hadn't stopped anywhere, we told them maybe we should stop for dinner. That bought us some time & eventually I said that maybe we should stop for the night since it was getting so late. They were so excited by the idea they really stopped asking questions. Probably helps that we have a DVD player in the van! It wasn't until we pulled on Disneyland Drive (how they didn't see the exit signs I still don't know, except they were really into the movie they were watching) that my 5 yr old saw a Disneyland bus & figured it out.
We've surprised them twice before. The year before that we spent a week in Vegas & surprised them with a stop in Disneyland on the way home. The drive is so long home from Vegas they didn't ask any questions. However my oldest knew something was up because I had an extra suitcase I wouldn't let them unpack in Vegas (full of their Disney clothes)! I do remember pulling into the hotel and my oldest asking if we were only stopping for Daddy for rest. Poor thing, does she really think we'd be that mean?
The time before that we flew & kept it secret for MONTHS! We drove to the airport and my husband kept telling them the car was just going there (they were little then). We were approaching SNA when the flight attendant (not knowing they didn't know) pointed out the Matterhorn to the girls.