I have tried in the past, as has DH, to talk to the grandparents about such.
Both the dc have December birthdays so the place can seem like Hamley's come January.
We've tried the 'can we all club together for one big present so ds learns the value of stuff?', we've tried the 'dc finding it overwhelming, can we stick to a couple of small presents each for birthday?', we've tried the 'ds wants X and is saving his pocket money towards it, shall we all help him?', otherwise they get tons of stuff that they wouldn't have picked themselves. It rarely works. If we ask them to scale back at the birthdays then they get double for Christmas. We have also tried the Amazon wishlists on the excuse that the family is so spread apart this will help guard against duplicate presents after one year when we got several duplicates - that is beginning to have a bit of an impact with all the other relatives except pils & paternal aunt who buy stuff from the lists and more.
I also worry that they are getting awfully materialistic and are beginning to expect presents all the time. I've tried this tack as well but to little avail.
However the clothes thing is getting better - at least now the odd t-shirt isn't too much of a problem but before, when babies and toddlers, they used to buy piles of stuff.
We do try to declutter a couple of times a year and ds1 is of an age when he feels proud about getting together a full set of whatever to take to the charity shop or sell on eBay etc. but the 5 year age gap between them doesn't help as ds2 hasn't outgrown stuff that ds1 has. Don't really have the answer, sorry 