Many children will struggle with letting go of anything when given a choice. Just seeing a toy seems to bring back its great value. It is a rare child that can truly get rid of some of their possessions.
One idea I’ve heard is to have a standing rule that before another toy comes in another toy has to be taken out. This seems like a great way to keep things where you want them, if you are where you want to be to start with.
I am not a real advocate of mom sneaking in and taking toys with hopes the child won’t notice. I want to be honest with my children. So when a large purge was needed, I told my children that I was going into their space and going to sort through their stuff. As I found things I thought could be discarded I would put them into a black bag.
Once I was done with my search and seize time, I would give the children two weeks. Within that two weeks they could reclaim anything they missed from their belongings. After two weeks, whatever was left was brought to a thrift store.
The two weeks allowed them to make sure I didn’t grab anything of real value to them. And anything that wasn’t noticed within that time period didn’t seem to have heart breaking power. There were even a few times when things were remembered and the child realized they were ok with it being gone. Although this may not have been a favorite time for the children, it was an effective decluttering tool.