This is a cute idea.
After your kids go to bed for the night, or while they are taking a nap, build a structure with blocks.When they wake up tell them you built them a special structure (house, building, thing) and they have to find it. Repeat as often as you like, but vary where you build and what you build. They will be excited to find what you have built for them! After they find it let them play with it, or knock it down.