I think you need to look at the entire cycle of the money.
It starts out as the parents’ in the form of Child Benefit.
£50 pcm is also the parents’ as is the interest.
If you take the view that they have made a gift of all of the Child Ben to the child (and there are no deprivation issues) then I think you are in the position where the £50.00 and the interest is income as well as the Child Benefit!
If you take the view that they are making a gift of the Child Benefit to the child less £50 pcm and any interest accrued on it I think the interest is income since it is accrued on the child’s capital and then transferred to the parents.
The final possibility is that the money belongs to the parents in substance although this is not the case in form. If this is the case then treat as their capital.
The first option does not really make a lot of sense but could exist.
The choice is really between 2 & 3 – If there are no other withdrawals from the account and the parents maintain it is the child’s account then I would be tempted to accept that as being the case.
What is the parent’s explanation for the situation?