Here's a little printable jigsaw for the kids to put together, with 3 different cutting templates. Choose the one you want, place it on top of the picture, and cut through both pieces at the same time. Or just cut your own pieces.
You could write a special message on the back of the jigsaw before you cut it up, too. Then, when the kids have put it together, stick the pieces together with sticky tape and turn it over to reveal the message.