When I started childminding, a very wise friend advised me to establish some rules about my own children's toys and sharing. She suggested these principles and they worked well for us:
All toys that are accessible can be played with by anyone.
If my own children had toys they didn't want to share, they were put away in their rooms.
Sharing meant that anyone could pick them up and play with them. It didn't mean they could take them off each other.
If one child appeared to be preventing the others from playing with a toy by guarding it or keeping it for an unreasonably long period, we would use a timer to help guide turn-taking. I was careful to make sure this didn't prevent children from deep exploration of a toy they were particularly enjoying.
Sharing can mean playing together with something, rather than taking turns. Sharing is not handing over a toy when another child demands it.
Depending on age and stage of development, I encouraged the children to negotiate, explain their own needs and feelings to each other and find a mutually agreeable arrangement. This equipped them to manage similar interactions when an adult was not around to mediate.
No child had to let someone else hold their comfort toy/transitional object.
Also, the majority of toys available were my toys which anyone could play with. This prevented my own children from feeling like they always had to share their own toys.
I watched an older child at a friend's house a few weeks ago using sharing to bully another guest. This girl brought out a box of toys which she was happy to share with a slightly older lad but not with his sister who was slightly younger. She was actively encouraging the boy to play with the toys then instructing the younger sister not to touch them because they were hers. Her mother did nothing to intervene.
I had already intervened in similar bullying over a game so didn't feel able to do anything as both girls had parents present who could have done so.
I think the adult analogy is a good one and not at all ridiculous.
You don't demand equal access to other people's property but it is socially appropriate to make certain things available to guests and friends who don't have their own to hand. You can fiddle around with the puzzle from my coffee table for as long as you like. You can borrow my phone to make a phone call but not explore it or keep hold of it. You absolutely cannot go and help yourself to the dresses in my wardrobe.
We wouldn't eat a bowl of crisps without offering them to others if it was the only one available but we also wouldn't offer food from our own plates during a meal, demand food from someone else's plate or help ourselves from someone else's plate. Those could all be described as sharing.
If I am with my friends, Jane and Mary, they both get to share the same things. If I encourage Mary to share my bowl of crisps but don't offer one to Jane, then Jane will, quite rightly, feel confused, hurt and angry.
There are complex rules around sharing and teaching them to children isn't simple. Some adults get the adult rules but can't work out a child-friendly equivalent, especially if it doesn't suit their own precious princess and a meltdown is likely to ensue.
I think you got it just right, OP.