I highly recommend the book 1-2-3 Magic for help with rule-setting with younger kids. It's short, succinct, straight-forward, respectful to kids and parents, and just WORKS.

Kids can be whiny, unhappy, entitled brats, or fun little humans that you enjoy spending time with. A lot depends on how you interact with them, and - importantly - how consistent you are with that.

I remember seeing once, in a big box store, a little girl having a tantrum with her dad over a trinket. The dad held out for about 10 minutes, and then gave in, and purchased the trinket. There is NOTHING that will incentivize bad behavior more than situations like this.