Baking board in sandbox, suitable board material?

[MasMats]

Cleaning day in the townhouse area on Saturday. Planning to contribute to the yard's sandbox. I'm going to make a table that can be used for the children's building, so not all works of art will need to balance on the frame around the box.
I was thinking of using formplyfa as a board material. Or is there something better that can withstand weather, wind, and children?
What suggestions do you have?

 

[Real7]

Formplyfa is good but expensive, and it might be a problem that it can become extremely slippery for a child wearing boots. At our preschool, they have a regular low table with an oilcloth fastened around it.

 

[vectrex]

Minerit is always available. But formplyfa is probably the best anyway. Better to replace it every 5 years or so!

 

[MasMats]

Passing by to pick up a sheet of formplyfa this afternoon. Can replace it when it gets too worn.