My parents used to go really over the top and it stresses me out because we couldn't match it.
I asked a couple of times if they could just buy one or two presents but the presents just kept coming. Eventually I just said I really appreciate it but I'm putting them away because we don't have room and I'll pull them out next birthday/Christmas so no need to buy for them as we had their stash already.
Honestly our house was like a toy store! Not to mention both kids are more outdoorsey kids so it was just a waste.
Could you maybe say you or your DH maybe say you just don't have room and you're also trying to teach the kids the importance of non material things? So maybe they could do 1 present and maybe another couple be a trip out with their grandparents somewhere fun?