Hi JW
Children who are adults over the age of 18 are not part of your household for Benefit purposes. So a grown up son or daughter could have as much money as £1000000 and it would not affect your Benefits.
If you pay rent you may be subject to a Non Dependent Deduction which is called a Housing Cost Contribution by UC.
David
Nothing on this board constitutes legal advice - always consult a professional about specific problems