This Tax Charge - note that they are not actually calling it a tax - impacts on anyone who (a) earns more than £50,000 and (b) their household receives child benefit. It is interesting to note that this Tax Charge is not based on total household income, but instead gets triggered if either parent (or carer) earns more than £50,000. Think about that - a family where each parent earns £49,000 (total income £98,000) is unaffected but a family where one person doesn't work but the other earns £50,000 or more is.
This applies to married couples living together, civil partners living together, an unmarried mixed gender couple living together or a same sex couple living together - that seems to cover all options although the guidance I have seen so far does not seem to mention single parent families (watch this space).
If either person has an income of £60,000 or more then they will pay a tax charge equivalent to the total child benefit they receive. If their income is between £50,000 and £60,000 they will pay a tax charge of 1% of their total child benefit per £100 above £50,000. So if they earn £55,000 they will pay a tax charge equal to 50% of their child benefit.
Just because you have to pay the tax charge does not stop you from continuing to receive the child benefit, but you can choose to opt out of it. However, there is a benefit to continuing to receive the money. Where the primary carer is receiving the benefit, they continue to get credits towards their state pension, even if they are not working. It is therefore worth continuing to receive the benefit, even if you just save it up to pay the tax charge at the end of the year.
If HMRC think you have missed this off your tax return, they will contact you.