Well, shopping at Argos helps, as does searching online for specific things they may have asked for.
My parents always told us that they wouild spend �20 on us for Xmas and we could have what we wanted so long as we didn't go over budget. Tight? Yes. But we always had other stuff from Grandparents. If you were really hard up you could do that.
But maybe you could start saving now? Put aside as much money as you can each week. Be really careful about not buying anything you don't need from now on. Don't buy the kids any toys etc from now until Xmas- that will make it all the more exciting when they do get what they want!
Also, if you have to, buy their presents on a 0% credit card so at least you are not paying any interest on it.