I'm a graphic designer so I have expensive software that makes this easy. BUT you really can make this yourself using other software. If you have a program that allows you to insert and crop photos, you're all set. This would also make a great non-digital collage (what would that be called---an analog collage? a traditional collage? Ha!).
First, photograph the artwork. Make sure you're taking pictures in a well-lit place. The front of my fridge happens to be near a sunny south-facing window, so I used that same magnetic clip to hold each piece. This time, not in a giant pile that's almost too heavy for the magnet to hold. Try to zoom in enough that you don't see the edges of the paper (or, in my case, the magnet).
(This one is my 3-year-old's artistic interpretation of the Angry Birds game!)
If you're not using a computer for this project, this is the step where you print out the photo of each piece of art, cut each into an equal-sized square (mine are just over 4.25"), and arrange in a grid on a large paper. Then affix them to the paper, and voila! Art, much faster than those of us who are doing this digitally.
I used Adobe InDesign to lay out a horizontal 24x36 page, and drew some blank square frames. I made the borders thin and black, but they don't need borders unless the individual pieces of artwork have a lot of white around the edges.
Then I placed (File > Place or Command+D) the artwork photos into each square. I rearranged a little bit, to balance some of the heavily-painted art with the lighter pencil drawings. Finally, I exported to a PDF (File > Export or Command+E) and ordered a poster-size print from 48hourprint.com. (I had a great coupon code that they e-mailed to me!)
The finished product! The frame was something I already had, but I believe it originally came from IKEA. (Note: This final image was altered in Photoshop because I'm planning to paint this wall and I wanted to test out my new paint color!)