Put them all in a wheelbarrow and have someone take a few laps.
Get two other adults to hold opposite ends of a sheet or blanket like a hammock, with the kids lying shoulder-to-shoulder perpendicular to the way an adult would lay in a hammock. The handlers can swing the hammock, bounce it, etc. to get the kids excited and generate the expressions.
If you can get above them, have them play ring-around-the-rosie, or use a hula hoop and have them encircle the hoop and hold onto it while they look up, and/or see if you can get them to all stand together inside the hoop.
Stack them behind each other on a playground slide.
Get 5 joint compound (or similar) buckets, put them somewhere scenic, and stick each kid in a bucket. Have them hide from you (heads down in the bucket) and all play peekaboo at the same time.
If you have enough adults, have adults hold each kid high in the air doing the "airplane" thing, with all the kids flying in tight formation. Frame the shot to get an interesting perspective of the kids perched atop a forest of arms.
I've never tried any of these, I was just trying to think of things that would keep the kids in a single, reasonably close-up frame for enough time to capture a few shots, while also generating some good genuine smiles.
Good luck, and post your results for us!