If you need the money to keep a roof over your heads and they have an income that will enable them to contribute, yes it's fair to ask (not tell) them to contribute.
If you don't need the money or they don't have an income, yes you are completely unreasonable in expecting them to pay. They are your child for goodness sake, not a lodger! You decided to have a child, you have to support them until they are able to demonstrate that they are willing and able to support themselves.
Charging your child rent "because they ought to pay me something" is utter bullshit. A child is not a financial investment! They will leave home when they are ready and able, when they are mature enough they will probably offer to pay some rent. Until then, try to look at them without seeing £££ symbols!