Might be controversial, but giving them a list to buy from might help if they follow it.
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Tell them you have too many toys each year, and would prefer cash if they wanted to give something.
We set up a photo stream with gift ideas for each kid and my siblings kids. Then add our parents in and they get to chose what gift they will buy and then my siblings do the same. It means that the kids are getting something the parents know they will like and use.
We set a budget, so this year we decided £60 max per child (there’s 8 children between us) and I’ll put 4 gifts (per kid) into the photo stream and my parents will chose which one to get. The other 3 gifts are then bought by my 3 siblings. Then they do they same for their children.
Our parents also give stockings and usually fill them with chocolate, colouring books, etc.
This has been working well for us for the past 6 years.