Our kids did not know where we were going on their first WDW trip. We told them it was a surprise.

We can drive to WDW in a day, but for the first trip we left late because of school and had to spend the night on the road. They asked where we were going but we just said it was a surprise.
At one point they guessed the beach. I thought for a moment and said there was a beach where we were going.
The hard part was having them NOT notice all of the WDW, Universal, and SeaWorld advertisements on I4 in Orlando. We got lucky in that they were reading or watching a DVD and not looking out side the car. The DW and I would point at the opposite side of I4 if we saw a dreaded WDW sign.
When we pulled off I4 into WDW I had to point out the WDW arch across the road. THEN the kids started hooting and hollering.
After they somewhat calmed down they reminded me that I had said we were going to the beach.

I told them they would understand shortly. Then I pointed at the sign at CBR as we pulled up to the guard shack.

Our kids were about four and seven so it was a bit easy to hide the surprise.
If this is the first trip to WDW I would think about holding on to the surprise as long as you can. The result is awesome.
After the first trip the kids know we are going and help plan a bit. However we still try to do something that is a surprise every trip if we can. On the first trip not only did they not know the destination they did not know that the grandparents would be at WDW as well.
One year we drove up to the mountains. They knew where we were going but they did not know the grandparents would be meeting us.

We did a bit of planning to make sure the kids did not see the grandparents car. We parked and went into the hotel. Sitting in the lobby, right near the entrance, was the grandparents. The kids did not see or recognize them at first.

Then the hooting and hollering started.
Next WDW trip, the surprise is going to one of the waterparks.

We have never been. Well I have been to River Country but that was a few years ago.

It looks like we will be at WDW for a couple of days and the first chance to go will be on a Monday. The plan is to get ready to go to the pool. When we get to the front of AKV, the kids will automatically start walking to the pool while we head to the bus stops. At some point they will tell us the pool is this way and we will say yes but we are going to BB. Didn't we tell you?

Surprise the kids when you can. It is well worth it. The kids still talk about our various surprises. If you cannot surprise them with the destination, surprise them with a character breakfast, Illumination's Cruise, finding paint brushes on TSI, family, animation class, etc.
We have a few more surprises in mind for future trips.....
Later,
Dan