We just surprised our girls, 5 & 8, last weekend. For us, telling them in advance would have been absolute torture because they would NEVER stop talking about it or asking when we were leaving. Additionally, the surprised look on their faces, which we video taped, was all worth it.
We drove from AZ to CA on Saturday telling the girls we had to drop daddy off for work, something we've done a couple times before because he will frequently go out of town to work. When we were in the Anaheim area, DH talked to DDs about the movie they were watching so they never saw the park or anything. We actually managed to get them into the hotel without them knowing where we were other than CA. After dinner we walked out of the hotel, we stayed at the Del Sol Inn across from the enterance, and oldest DD started looking around and reading things and realized we were there and started screaming and so did our little one. Their faces lit up, it was great!
We also surprised them with breakfast at the Plaza Inn with Minnie & Friends the first thing the next morning. (HIGHLY recommend this breakfast BTW, we met 9 characters and since we got there first thing we saw some of the characters 2 & 3 times!!!) The next day we surprised them with lunch at Ariel's Grotto. (Again, great experience, 4 princesses and the fairy godmother.)
The whole trip was awesome. This was our second trip there and second surprise trip. Maybe next time we'll let them plan but watching their excitement was priceless.