I disagree that “lowering expectations” makes you sad and downtrodden.
In fact, I think you need high self-esteem
to be capable of chilling out about this kind of thing.
If you have high self-esteem, you seek out people who value you and treat you well. You choose people who meet your high expectations.
However, you don’t get to choose everybody who’s in your life. Family, for example.
If you have high levels of trust, you will believe the best of these people. You will cut them some slack, assuming they have their own sensible motives for what they do, even if it isn’t obvious what they are. You will feel happy believing you are surrounded by nice people and they will be nicer people around you, reacting to your trust in them. And you would probably be right in your assessment, given that most people most of the time believe that their own motives are good.
Your high self-esteem will allow you to let them live their lives their own way. You will be self-sufficient. You will not expect money from them because nobody has the right to expect money from anybody else (apart from when parents are bringing up children). That is what having low expectations is all about: not hanging your hopes, and your happiness, on things that might never happen. Being a bit more easy-come-easy-go.
If they do actually disappoint you, you will emotionally disengage, rather than give them the power to make you unhappy. So, in this situation, you would stop trying to persuade your children that their grandparents love them all equally. It is apparently not the truth and you’re getting yourself tangled up in something that probably just needs to be accepted as a fact, or forgotten about. It needs to become less important.
When you let their behaviour make you this unhappy, you are letting them dictate the agenda, which is surely not what you want. Your family doesn’t need to be upset by their odd choices.