sub.Select(role=>new RoleAccess
{
id = parent.Id,
name = parent.Description,
children=child
});
"children" is a list and you're trying to assign it a single item. What you need to do is make a list with a single item in it and assign that to children. You do that using code like this;
sub.Select(role=>new RoleAccess
{
id = parent.Id,
name = parent.Description,
children= new List<ChildRole> { child }
});