It's a big moment in a parent's life when the children leave school – not least because it often comes with a loss of benefits or tax credits. On the other hand, another working adult in the house can often help soften the blow. For those with children still studying after GCSEs, though, time's running out to make sure you're not losing money you're still entitled to.

When HMRC starts asking you to update your information, it's always wise to listen. Right now, the call's going out for parents whose children just got their GCSE results. From this point on, those kids are free to leave the world of compulsory education and join the UK's workforce. If the taxman doesn't hear differently from you by the 31st of August, that's exactly what he'll assume they're doing. If he's got that wrong, though, it could end up costing you a lot of money in lost Child Benefit or Child Tax Credits.

Unlike most HMRC deadlines, you aren't suddenly going to get lumped with a painful fine if you miss this one. You will, however, quietly lose your entitlement to benefits and tax credits you've been used to. If your child is moving on to find a job, that's fair enough. However, if you've got kids continuing their full-time education after their GCSEs, you should still be getting that money. It comes down to the idea of “approved education or training”. Basically, children aged between 16 and 19 can still qualify for Child Benefit or Child Tax Credits as long as they're doing one of the following:

•    Staying in full-time education.
•    Taking an approved apprenticeship or training course.
•    Studying for a vocational qualification.

If you haven't updated your children's educational status yet, you've got until the 31st to get the taxman's facts straight. After that, he'll assume you're no longer eligible. It needn't be a big hassle to get it all sorted out, if you're quick. If you've got a Personal Tax Account, just log into it and update your information. If you don't have an account, you'll need to jump through a couple of extra hoops. Make sure you've got a payslip or P60 handy, along with your National Insurance number. You only have to do the initial set-up once, and you can even do it on your phone.

If you can't get online, you can call HMRC on 0300 200 3100. It's probably a bit late to try sending a letter, but in a pinch you might at least be able to limit the money you'll lose. The addresses you'll need are:

•    For Child Tax Credits: Tax Credits Office, Preston, PR1 4AT
•    For Child Benefit: Child Benefit: Child Benefit Office, PO Box 1, Newcastle Upon Tyne, NE88 1AA

If your cash could do with a top up use our tax refund calculator to get an instant estimate of how much cash you could be owed.  We've been in the industry since 1999, making us the UK's leading tax rebate and tax return experts.