Parenting

Incorporate it as a lesson on boundaries. If something belongs to someone and they don't want to share it, that should be respected.

There doesn't need to be an explanation beyond that. The more a kid is taught about boundaries, the easier it'll become. The kid's boundaries should also be respected when they state them (provided that the relevant situation is safe, of course.)

I'm not a parent, but I work a job where I teach boundaries to kids every day. My students are all on the autism spectrum, with different skill levels for pretty much everything. The kids have to learn not to touch other people. They have to learn that they can't eat another kid's snack. They have to learn that if someone else brings in a toy from home, it's okay if they can't have a turn with it. There are a lot of things we teach, and no two kids have the same programs, but boundaries are taught to everyone. "I need space" is a common phrase to hear.

Alternatively:

Offer the kid something else. Pick two other things they like and offer a choice between them. Say something like, "This drink is Mommy's/Daddy's, but I can give you some Tic Tacs or a piggy-back ride to the car instead." Or whatever the kid likes and you're able to give them. It's not going to be foolproof, but it does help assuage the sense that they're missing out on something that Mommy/Daddy gets to have.