Get 3 tubs and ask your children to fill one tub each with the toys they want to keep. You can guide them a little in the process - encourage them to keep the leggo and chuck the battery operated stuff
Then take the rest to charity. I throw out nearly all the toys that MIL gives us because they are rubbishy junk. I don't feel guilty about chucking them. I figure she should feel guilty about filling my home with plastic that is bound for landfill. DH and I are thinking of asking our families to stop giving toys as gifts. Clothes, books and money will be fine lol.
Anything still in the box could go to those trees they have at K-mart, target etc for underprivileged families.
We cull pretty often so now the boys are used to the concept of their toys going to people who don't have any. I try to keep it to 2 boxes plus a few big pieces so when we get toys in other toys go out.