For me, addressing these kinds of internal concerns has been a matter of becoming really clear and comfortable about limits (what I am and am not willing to do, provide, allow, buy, give, permit) and about my children's feelings in response to those limits. And getting more comfortable with their wanting (and expressing their wanting) being okay.
The upset (disappointment, complaining, arguing, reasoning, bargaining, pouting....) that may follow a limit is kind of their personal process of struggling with something they can't change, trying to change it, and ultimately coming to terms with it. It sure can feel like ingratitude or entitlement (and I'm pretty sure if I expressed these kinds of things, my own parents would have responded with strong disapproval and no acceptance, which is probably why my kids expressing those sorts of feelings is so triggering for me) but I do believe it's more of an existential thing. When I can see it as acceptable, as "not wrong," (including tolerating my discomfort with it and my upset feelings about it!), it is helpful to the process and to the overall dynamic.
When I can have my feelings, and they can too, life gets a whole lot easier.
Taking my resistance (of their reactions, of their open expressions of wanting, of their apparent "insatiability") out of the equation has been the key thing for me.
My daughter recently expressed some thoughts and feelings about our family having an iPad recently (why don't we, why can't we, wouldn't that be great, etc.) and I really had to get conscious of how triggered I was, and how resistant of her I was, in that moment! Because I was profoundly irritated and annoyed. I did a quick shift, and just invited her thoughts (her teacher has been using her own iPad in class sometimes, it's clear that other kids were familiar with it/have experience with it, etc. etc.) and let go of my anxiety and defensiveness (which also was happening for fully valid reasons....my own triggers) so that I could be present with what was actually happening in that moment.
I don't push a lot of distinction between "want" and "need." Wants tend to be strategies to meet needs, anyway (the need for fun, stimulation, challenge, etc.) Recognizing the expressed want or wish as fully legitimate and valid (whether or not I am able or willing to grant it, and whether or not it reassures me or triggers dismay/doubt in me as far as how well my kid is doing) is a clear way of taking my own resistance out of the situation. I can be present with their wanting, without trying to "instruct" them about how it's not a need (essentially that they "shouldn't" want that!) This is a way of giving them room to be who/what they are & feel in that moment, and it gives space for reflection, for change, for process. We're all less likely to get stuck places if I recognize the process model of life, rather than feeling like I need to guide all these processes (if I don't want them to end up entitled, self-absorbed, antisocial, ungrateful, selfish....)
We all make mistakes of some kind. It's quite possible to, out of anxiety or something else, begin to confuse loving with giving things/providing services so that a child learns to equate love with getting what they want. It gets painful when those same kids escalate their demands, and don't seem to appreciate things OR to be in touch with reality ("You never give me ANYthing!!") To me, this simply is a reality to engage and own, and to which to respond constructively. The dynamic happened for a reason; I can be compassionate with myself for the "mistake." If I own my disappointment and my anxiety/doubt about the dynamic that has resulted, then I am less stuck getting reactive and resisting the child's communications as wrong or unacceptable. Those behaviors, attitudes and communications are less likely to trigger feelings of powerlessness.
In terms of a "constructive" response: I can recognize that I may need to pay attention to my own needs and my integrity, and to honor & express my personal limits responsibly by speaking personally ("I'm not willing," "I don't want to," "I want to," etc.), so that I am bringing myself to the table now rather than creating the impression that my needs/limits don't matter or even exist! It's just a matter of adjusting how I relate, and how I tolerate my anxiety (recognizing that I don't need to buy/give at those times, anymore.)
This is something that, over time, children can & will respond to. Seeing that upsets or protests in response to these changes make sense and don't indicate anything (other than honest, authentic reactions!) is helpful, too.
When their wants & feelings of upset/disappointment feel like an indictment of me, that's when I have problems and begin to resist them, show irritation/disapproval, and pressure them. And also, feel like I need to instruct them or enlighten them.
Increasing my toleration for their wants & their feelings (increasing my ability to stay emotionally regulated in those moments) has been the most helpful place to put my attention.
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