I would love to see Charlie at the kicking business. I'm not sure if it's an attempt to dig a tunnel into it (hitting hard plastic fairly quickly), or if it's a kind of dog type thing to kick sand over the bit they just pee'd in and cover it up - maybe a bit of both! Although you think they'd learn quickly that the sand doesn't go down very far. I use this Chinchilla Sand. I usually get it from Viovet where it's not too expensive and you don't have to spend £20 at Amazon to get it as an add-on item. It's not cheap, but I just use it for the potty now so it lasts well and doesn't work out too expensive. I gave up having a sand bath for Charlie because he didn't use it (kicked the sand out and turned it into a cosy second nesting area).
Supreme Petfoods Science Bathing Sand 1.5 Litre: Amazon.co.uk: Pet Supplies