When a child becomes 16 he changes from being a child to a young person. Children automatically qualify for child tax credits in the hands of the parent. Young people are only qualify provided that they remain in education (and that ceases after a few more years). By default, unless you TELL the tax credit that the child is now a QUALIFYING young person they will assume not, and hence stop the award. The transition from a child to a qualifying young person is a notifiable change of circumstances. It looks as if they have now accepted that this is the case. Working hours are irrelevant to qualifying for CTC (but are relevant for Working Tax Credits which are dealt with in the same claim).
The first £300 of joint household investment income is exempt from the income to be taken into account in assessing the claim. But capital is disregarded. Only the income arising.