For context, I am childfree so not always sure what normal expectations are for kids because I have limited frame of reference.
I have two nieces by my sister, age 6 and 9. They are the only kids in the family and get lots of attention and spoiling (but nothing excessive) as a result. In the last year or so they have become really entitled imo. They never say thank you for gifts, treats, trips out or effort made for them, and they will openly ask for things they want from me and other family members. Last weekend I went over for the 9 yo's birthday and took gifts and cupcakes for her, and she didn't say thank you, even after prompting. She asked for her present as soon as she saw me and then didn't engage after that. At Christmas I gave her little sister a gift worth about £50 and a card. She opened the gift and then asked me if there was money inside the card (there wasn't!). Other times I have seen them recently they always ask me for things - treats, toys that they see and want - and at the same time are quite dismissive of me.
There are lots of little behaviours like this and it's really putting me off spending time with them, but I feel like as an adult I should rise above it because they can't be expected to know better - just wondering if others would agree, or expect more of them? None of my friends' kids behave like this - they aren't angels but they are always grateful for any gifts, treats etc., and if they aren't their parents are mortified, whereas my sister and BIL never really notice or pull my nieces up on this.
YABU - all kids do this kind of thing at some point, don't overthink it
YANBU - it's rude and not the norm