I would not even ask a friend for money towards a Christmas dinner that I was hosting.
Either I could afford it, or I couldn't.
If you can't afford to host realistically, how about scaling down the Christmas dinner so it doesn't cost over £45, rather than asking them for money?
The problem I have with people who ask others to contribute to meals they are hosting, is that often as the guest (whether I'm family or not), I've had zero input on the actual food being served. The host has been able to choose what foods are on the table, how much of each, and there's every chance I'm not actually going to like what's being served. I don't get a say and am expected to contribute to that. It's just not how I'd choose to spend my money.
If my mum asked me for money towards Xmas dinner, I'd tell her to leave it, personally. I'd rather spend the money on having the food I wanted, the way I wanted.
That being said, I'd be likely to offer my mum money towards it, if I knew she was struggling. Either your children are a bit thoughtless in this regard, or they're completely unaware that things are tight for you.
I'm a bit confused though because you say things aren't that hard up, but then thinking of asking a collective £45 from them all. Are you just doing it on principle? If that's the case, YABU and that's wrong.