If they have Nokia phones you can get the Counters app which, as its name suggests, counts such things. It can't yet do automatic cut offs or anything like that though...

(p.s. have you considered whether Pay and Go may be a better option - at that level of data usage it would be competitively priced to a contract with the added benefit that you can't get a nasty surprise on the bill at the end of the month. It's a low tech solution but very effective (plus if they want more data they can easily just pay for more). Also some networks (Tesco, for instance) will cap contracts. ie. they cut services at their end once the monthly bill reaches a certain level.)