Prosaic, my opinions: Current societal acceptance of materialism and consumerism is such that presently only a small minority of adults come to realize that one can have more enjoyment or fulfillment by having fewer things. I suspect that many adults don't have the mental capacity to come to this realization on their own (though perhaps they could follow others-- if they lived in a society that was made up mostly of people who tried not to acquire things). Additionally, some who do have the capacity to consider the issue will actively choose to acquire more rather than less.
Thus, it may not be possible to get all of your children on board with the decluttering. I'm not trying to insult your children; some may not be at a stage where he/she can understand your point of view. As an analogy, consider that an anti-materialism/anti-consumerism message coming from one, or even both parents, set against a backdrop of ubiquitous competing messages from almost all aspects of society is somewhat like you telling your kids to believe that the world is flat. Alternatively, perhaps he/she may fully understand your message, but reject it nonetheless. It may not be a battle you can win for all persons. It would surprise me if you could get 4 out of 4 kids to declutter voluntarily.
Some of your children may think similarly to you, and thus if you can expose them to the kind of experiences that allow you to feel secure, or even empowered, with fewer things, you may ultimately help them understand.
For me, I think my acceptance of fewer things came from my exposure to camping and outdoors activities, and exposure to poverty and the underprivileged. I was never poor myself, so exposure doesn't mean that they have to be poor.
Involuntarily throwing things away, might lead to some pretty hurt feelings. I only have a very young child, so I don't know if I'd try the following with an older child or not, but I have wondered how a person might respond to this strategy:
You remove a number of items that seem to be redundant or useless while the child isn't home. You inventory each item removed for your own records. When the child returns, you explain what has happened, and explain that none of the items have been thrown away, just moved to a different location. Each child is given an opportunity over the next 3 or 6 months to ask for any of the items back. If the child can describe the item and wants it back, you give it back, without question. The child doesn't have to justify why he/she wants it back, the child just has to be able to recall that he/she actually owns the specific item. If a person cannot recall owning an item over a 6 month period, it is less likely to have been an important posession.