Building a battlefield in a sandbox.

[OG SCE]

So,  i recently took my young daughter to a kids museum and in one room they had a number of sandbox tables for her to play. They each had some form of loose basing materials with which to interact.  They seemed like an interesting way to build dynamic terrain for smaller games or perhaps warcry.  My daughter could also join in with Warhammer doll-play - shes still way too young for rules, but could play like a doll house.

I think the choice of terrain building materials will important.  Has anyone any experience with these kinds of tables. Kid friendly play-sand is generally too fine and unstable.  What else could i use?

Could i just pick up a variety of natural materials from a local beach or forest trail?

All advice appreciated