I set up 2 long 2by8s on top of crates. I sat on the grass (with camera of course) and watched. Which I often do...sit back and watch. The kids don't need me to tell them what to do, because if I did...they wouldn't learn it for themselves.
They used about 10 more peices to create a 'boardwalk' through the sand box. The kids were so engaged that they were having a hard time leaving when their parents came to get them. The next day they were excited to go outside and work on their project.
The longest board that runs along the back of the sandbox is nailed down, but all the other boards (minus the two I started) were set up (and down and up and down and up) by the kids.
Your children were being creative and using their noggins to figure out how to solve the problem of creating a path that would go all the away around the sandbox. They had to figure out how to balance the boards and where to place them and which sizes would fit in which spots. After they figured all that out...they needed to listen to their bodies as to how to balance while on the boards because some of them were rockety.
They would fall....and get up...and refigure...and try again.
Your children were working together. They were communicating with each other to fix a problem. They were helping each other to get across or when they fell off, they were helping each other get back up.
This, right here, is what is most important in your childrens learning. Compassion. Problem solving. Helping each other.
I love watching them and how excited they get when they are creating something fun together.