Also, we learned to never negotiate. We presented options and let them choose.
Kind of related...when the kids were little, I’d come back from trade shows with bags of swag and trinkets. If I gave it to them according to what I thought each would like, everybody was upset, mad at me, and lots of tantrums.
I solved it by dumping all the treasure on the floor with simple instructions. You each pick out one thing you like and we’ll go around the circle until everything is gone. Then you can each make two trades that you mutually agree upon with your brother or sister. BUT, if I hear any whining or complaining, I will take it all back and put it in the trash. It worked like a charm. Kids all learned how cooperation could lead to better outcomes for all and I never once had to dump it in the trash.