It kind of depends on your relationship with them, but what I would do is plan to eat at home. When they bring up the subject of going out, would you be comfortable being honest and saying "You know, money's kind of tight for us right now, I'd be happy to make dinner at home" They'll either offer to buy for you and the girls or defer to you preparing a meal at home. The down side is that you have to be prepared to cook for them if they don't want to pay for dinner.
I like luckylily's advice. What more special than to prepare a cooked meal at home. What if you tell them that the girls really want to cook for their grandparents? Something simple like spaguietti (sp.?) and salad. HTH