As a grandmother, I spend alot of time thinking fondly of the days when my kids were younger (me too!). I don't actually remember them sitting still long enough to build sandcastles - nor would they have willingly played together so quietly - but hey, in my old age I get to fudge on some of this stuff...
As with the majority of my sculptures, this one is made entirely of polymer clays, with wire & foil armatures. The children are permanently attached to the base which measures roughly 4"x6". The top of the base is also covered in polymer clay (have you found the crab?). The bottom is covered in felt to protect shelving and furniture.
