That's a good, fun, idea KatieJane. (Incidentally, where do you get those sort of bags from?)
Instead of the dead sea mud, DD, you could always use clay. It'll be basically the same (it's all micro-fine alluvial deposits of one form or another, I suppose) but probably much cheaper (and available in industrial quantities!). I use modelling clay (which is the light grey creamy stuff you see the AllWam (and others) girls wrestling in) but they have a huge range of types and colours. What you want is something very 'plastic' with lots of 'slip' and no grog (sand) in it. You can get it in 12½Kg sacks for a few quid, which will last you for quite a while!
For details (including rough purchase price and availability as well as tips on how to mix it and how not to block the drains with it!), see my article on filling up your tights with sloppy clay at the link, below.
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Claymore_Wam's "Useful Info" Pages