Ignore the posters saying they wouldn't take anything op
It's not about if you can afford to have them there is it.
It's about an independent adult earning an adult wage and learning about real life 
Why shouldn't they pay their way??
Yes things are expensive now but it's no different to when we were younger.
I paid keep to my parents on an apprenticeship and gave them 25% every month.
I also had to pay may own phone bill and travel and save and have a social life. It was bloody hard but the further into my career I got the more I got laid and eventually in my early twenties moved into my own place.
I knew how to budget and what was classes as a need and a want which a lot of people now adays really have no idea about.
I've worked my absolute arse off and am reaping the benefits and it's thanks to my parents teaching me to budget and have a work ethic.
I have done the same with my dc.
My eldest was helped with a first car and money towards insurance for the first year then it was down to them.
Same with the next one.
Once they were out of training and into a full time job they have contributed 20% to the household.
It gets put into two separate pots.
1 is into the household account and the other is for things like recently young adult ds needed a new bed and mattress so it paid for that
Their phone bill also comes out of that account.
I don't see the issue with them paying towards their living as it teaches them