Could you start a tradition for what they get, if your kids are still little and this is a common occurrence? Something like a book, an accessory and a pack of their favourite sweets, and then some money into a savings account? Or a nice outfit? Would eliminate the year on year building, and mean if they're not sure they can use their time with the kids to get them to choose.
If they have your children once a week, can they not simply be reassured that you're happy with anything, so to get whatever they'd like - you're happy to store it, don't mind what brands and materials etc? (This was the worry for some relatives I know)
Are your children old enough to write or dictate a list? If so, could you just allocate grandparents a present off it (I have a friend who's mother still does this and she's 30, with siblings aged 28 and 32, and her xmas list is still due to her mother by Halloween!)
Could you just give a general category of what they're into?
Could you let them know what you're buying, and let them buy things to go with it?
(EG we're buying a car garage, so Freddie would love some cars and trucks to go in it; Sam's getting a dressing up box and a couple of costumes, so you could find some accessories and other outfits; Jane is really keen on lego at the minute, we're buying a big box to get her started, but I'm sure she'd love any kind of lego set; we've been doing a lot of board games recently, so would love some to add to our collection; Alex wants an easel so we've got one and a big role of paper, could you get some arty stuff to do with it?) - Hopefully shouldn't require much additional though, but will help guide grandparents who aren't sure what to put.
Also, feel free to pass this onto your partner, and say 'your parents want to know what to get the kids for xmas, can you figure something out? Read this mumsnet thread for some suggestions!'