Sorry but some of the posters on here seem to live enchanted lives and need a reality check. Just because you wouldn't, doesn't mean she shouldn't if you take her circumstances into account.
Yes she choose to have kids, but unexpected things happen, income changes and no one can predict family members with health issues which means they are on long term benefits. With that comes extra responsibilities for the parent permanently, not just at Christmas.
She is struggling, this is not a normal Sunday lunch type meal with Chicken and veg, Its Christmas day, all the trimmings, drinks and ...yes...treats and the exchange of gifts.
What's wrong with aspiring to make it a lovely day for all and asking those with money (which includes the 17 year old with a substantial disposable income) from being asked to contribute...IF THEY CAN AFFORD IT - that was all she asked. A single guy with a decent income can afford to give his Mum the money he saves from providing for himself that day. Her Daughter and Grandkids, too - £15 is not much in the scale of things.
She makes it clear she has been funding it herself for years and has never asked before, read between the lines folks, she is giving her all and this year to celebrate in the way they are used to, she is struggling, yes she can economise and yes, she has made it clear she will if they cannot afford it, but how long does this go on for. Does she fund Christmas for eternity??? Not everyone has that luxury.
The 17 year old, has issues and struggles with shopping and lets be honest, kids (whether adult kids) will just bring what they want and not necessarily what is needed. It just makes it easier for the hostess to know what is coming by getting the money and managing it herself.
Some people (and I am not saying this is the OP) put themselves in debt to pay for Christmas presents and end up paying it off all year. I'd be mortified if I knew my Mum was struggling and I hadn't though to contribute.
And yes, my parents divorced and I was brought up in a single parent family. Three of us were brought into a world with the expectation that we would have two incomes in the house, but sadly that did not transpire. My Mum worked and brought three of us up, frequently putting us before her own needs . When I started working, the housing benefit she got (which was the only thing she got) was stopped because of my income. She simply didn't have the money to cover it, I had to move out or cough up. That was the reality of being on the breadline.
And actually, it taught me, at an early age, about money management and prioritising. Mum helped me when I was short and I helped here, that is what families do. Yes, I would have loved to live for free at home, living the high life, but not if it meant my Mum struggled.
Even as a mature adult, if my elderly invites us over for a meal, I offer money or take the joint of meat etc. She only has a basic state pension and she would turn off the heating until we arrive so she has money to treat us if she thought she could get away with it. That is what Mums do. But when there is money available, elsewhere, then the burden needs to be shared.
I think the OP is a pretty amazing woman, still wanting to provide for her kids even as adults. Big Hugs OP - as I said before, follow your gut instinct, they are your family. Have a fantastic Christmas. x