I so came on to say WTF, however the statement (investing it) stopped me. However.... The job of a parent is to prepare our children for becoming a part of society (middle class perspective - real perspective is how to raise our kids to be an adult without parents funding your lifestyle). However, I do believe you're being too simplistic and revealing a privileged lifestyle. Without going back to the Victorians and older the idea that our kids are entitled to a full education and a certain lifestyle is a very very modern concept AND one that is restricted to individuals with a healthy income. It was seen as beneficial to have multiple kids to help out (be that with the harvest or paying rent). 1000s of years of families would laugh/be gobsmacked at the idea that demanding rent is immoral. Your question is not only upper middle class but also very post 2000 parenting. Prior to recently (and even now this is reserved for parents who have big inheritances or earn well) kids were an asset.
I love the fact you're sitting back, having lit the touch paper, and watching the responses - good on you, but it's only those in a privileged position who have the capacity to state its immoral, who else would/could even think this! The underlying, generic, issue is that we, as parents, have a duty to raise our kids to interact in society to the general benefit of that society - regardless of whether we realise that or not. However those of us who have, have no right to judge those who have not.
I grew up in a distinctly working class environment (though my mother would deny this to her grave) but my mother clawed herself up and out of that (following the 'American way') and is now considered middle/upper class and woe betide anyone who questions her roots! I am, of course, left conflicted (a lovely health visitor I once encountered remarked that its mainly middle class mothers who suffer from PTSD because only middle class women have the time - Upper class have nannies - end of!), and have strived to raise my kids to appreciate all. I went to Uni (to not do so would have meant being thrown out - it was go to uni and do well or leave home) but did middling. But she was on the rise so pretended not to notice. My point here is that you're blatantly well off. My mother enabled (+ grants, + student loans) my education and yes I've built on that loads, but believe my own kids are too entitled! They don't believe that I, or my DH (their dad) were ever considered 'poor'.
Sorry for the monologue, but it was important! As parents, regardless of our monetary social status, it's vital that we raise our kids to go out there and be ready to stand on their own two feet. So NO, I seriously don't think it's 'immoral' to charge a working 'child' (cos they have to probably be 16+ and more likely 18+) rent. In fact I'd turn it around - I believe its immoral NOT to charge them rent because, 'what way are you teaching them about real life?!' We need to teach our kids to stand on their own two feet. So in a nutshell - yeah - you are being TOTALLY unreasonable!