I don't know much about this game but I'm sure it'll keep the kids entertained and if you were looking at purchasing a DS game anyway, you'll now receive Hamsterz for free. :)
The best instock price I managed to find was about a tenner from Mymemory.
Let everyone know if you find any good bargains for DS games and I'll add it to the OP.
International Track and Field is the cheapest game to take advantage of the offer, coming to under £6 and then you'll receive the free Hamsterz game too.
Hope that helps. :)