As every mother knows, resentment burns no hotter than at the Gates of Hell, aka the school gates.

And one issue that burns hottest is the 'lucky' parents luxuriating in free childcare from willing, nimble grandparents.

Those who have to pay a king's ransom for a child minder or a nursery and worry themselves silly about the cost – financial and psychological – of leaving their children with 'strangers' when they have to work, loathe the privilege of the parents who get off those bills scot-free.

And I can see their point. But that's life.

Drive past any primary school at home time and it's like a Saga gathering – there are more over 60s waiting to collect children than parents. It's a sign of the times.

This might irk mothers and fathers whose parents aren't willing or able to care for their grandchildren, live hundreds of miles away or died before they could even meet their grandchildren.

But we rarely think about how fed up and even resentful those grandparents might be.

They are signing over their well-earned years of retirement to potty-training, repeat-reading The Gruffalo and having their sitting rooms invaded by Crayola and Thomas the Tank engine when their friends are enjoying peace and cream teas in the sunshine in Burnham Market.

As much as they love their grandchildren, having to look after them for days at a time can't be all pleasure and fun. One of the joys of grandparenting is to spoil and indulge then give the children back when they get crotchety.

The bringing up bit is tough for grandparents who have done it all before with their own children.

But anxious to help – and keep in with – their children, who, in these stretched times need two incomes to pay a mortgage, run a car, and in many cases still paying off student loans, these grandparents don't want to say no.

As one grandfather said to me: 'We've paid off our mortgage and are far more comfortable financially than my son and his wife will ever be. We are the fortunate generation who could take early retirement on a full-salary pension and have had it easy compared to what our children face – working until they are 80 with limited pension.'

So guilt and a strong social conscience make them volunteer. Bravo for them.

Grandparent childminders are the unsung heroes of our time and deserve a great big fanfare of gratitude and recognition.

They are an essential factor in keeping the economy going by making sacrifices of time and money to enable women to go back to work.

These grandparents are history-makers. Without their sacrifice, so many women would be unable to go back to work. Their service is invaluable but largely overlooked in nation's workforce development.

This week it was revealed that almost two million grandparents in Britain have left their jobs, gone part-time or cut their working hours to make time for childcare. That's a huge sacrifice to help your children, but the power of parental feeling is so fierce and enduring, most parents put their children, however old, before themselves.

The study by Ipsos MORI commissioned jointly by Grandparents Plus, Save the Children and the Family and Childcare Trust, also found grandparents had even taken sick leave to babysit for their children.

It might be hugely generous and philanthropic but it is also another step in reviving an old-fashioned extended family and unity between the generations – a throwback to the days when grandparents would naturally look after the grandchildren and families helped each other out, creating special bonds between grandparents and grandchildren which can only be a positive thing for everyone.

It also gives grandparents a second chance to parent differently from their first time – but most of all, it's a huge privilege to be trusted with the care of grandchildren.

Sadly, it's unlikely to be a privilege my generation will be able to enjoy because we will have to work well into our 70s.

That's when a crisis in childcare will hit – when the hidden childcare army of grandparents are stuck in the workplace with their own mortgages still to pay.

So, to the grandparents making this sacrifice for my generation every day, we salute you.