DD is definitely one of those kids who wants to keep all of her toys, down to little plastic pieces of trash. For years, this has been a struggle. When she was younger, I could have DH take her somewhere and I'd declutter it. But, while that hasn't caused tears (she didn't know what I did) certain toys did get asked about sooner or later. I didn't like this method, it seemed dishonest, and admitting that I threw away something she was asking about also seemed really lousy.
When she was 4 or 5 I started trying to get her cooperation in decluttering, allowing her to choose. That didn't work well either. It didn't upset her but I'd get very little decluttered. She'd give me about 6 toys, 4 of them clearly trash anyway. (Out of hundreds, maybe thousands - we have a BAD toy clutter problem even though DH and I only buy her 2 toys a year. It's the extended family).
So on Sunday, I was going through her books and made a pile of books that she has really outgrown. I showed her the pile and gave her the chance to look through all of them again. As I feared, her first impulse was to say "I want to keep that one!" to the first one in the pile. I knew where that was going, so I said she could pick out 3 and keep them. That made her happy. She actually ended up with 4 but I was satisfied. I got to declutter a good pile and she was happy.
I told DH about how that went and asked him if he thought I could do the same with the toys - I sort them out but give her a chance to choose a few to keep. That places the emphasis not on her choosing toys to get rid of (and giving her a quota once was absolutely painful) but ones to KEEP. So we tried it. I filled a huge bin full of stuff I didn't think was worth keeping, things I didn't see her playing with anymore. Most of it so worthless I don't even plan to freecycle it though I'm one of those tree-huggers who can't bear to throw stuff away. I gave her the bin and told her to pick out 5 things to keep. She did. It went really well. She didn't challenge the premise of the exercise, didn't even challenge the number I gave her. She picked out 5 items. The last one she spent a little time at, choosing among a few possibles. But no tears. I was even prepared to be a little flexible with the numbers. I would have happily kept, say, 8 items as long as we could get rid of the box and keep her happy.
She is now really excited that her toys are organized. And I think this exercise kept our trust strong, too. She didn't have to patrol me while I went through the toys, because not even one broken plastic doll arm went into the trash that she didn't review.