"You have nothing." <-- This is something we've taught our kids (4 and 6) over the years. Of course we would never phrase it that way. In essence, it's the idea that they are part of our family, which implies a social contract. Mom and Dad are the boss, and that's non negotiable. Their toys and possessions are only theirs in the sense that we provide them, and we reserve the right to take them away as we see fit. They are entitled to love and respect and essentials, but nothing non-essential. We pay close attention to what toys or other items are of high interest since we leverage these when meting out discipline.
The idea that possessions are conditionally owned as part of a social contract is something kids need to learn in preparation for adulthood. Even if you own your house outright, you never own it unconditionally, since it only remains yours as long as you pay your property taxes. In fact all assets exist within a legal framework, and governments can (and do) confiscate assets in various situations. Better for kids to learn this from loving parents than to learn it from society as adults.