My kids are small, but my own mum asked me to contribute £50 a week from the age of 18. I was studying locally then, but my boyfriend (now DH) has started working (earining probably £50 a day, self employed) so it was a shared contribution. We had our own room and bathroom, and I thought it was fair. When I was older, and got a job, I moved into a annex with my boyfriend, which my mum owned. It wasn’t done up to rentable spec then (it was OpenPlan for example when the kitchen needed to be enclosed to be legal do health and safety) so she wasn’t losing out on rent, but I covered heating/electricity and council tax etc, and obviously all my own food etc)
I was working by then, but earining close to minimum wage, and BF was in and out of work due to the recession. We were also very wasteful and didn’t learn how to budget, was always in our overdrafts, had lavish holidays etc.
My mum didn’t need the money, she had lots of property and is very wealthy, and what she should have done (in my opinion) is to take a larger chunk of my wage from me/us. She could have kept that as a deposit for a house or something.
We were adults and should have just learnt, but the only way we have really learnt the value of money is to have our own house/mortgage and live through it.
If it’s a long-term thing due to the location of their work, I would say they should definitely contribute a meaningful figure you can keep aside for them if you don’t need it, or use a portion of it for food/bills.
Or, better yet, get them to set up a savings account to direct debit over a chunk for savings (they’ll never get it so good in terms of being able to save) and a small one for you to cover bills:food for you.
If they’re four young adults, all working, have they thought about a flat share in London? It might be slumming it a bit but they must want their own space?