Brothers Silas and Isaac Sinclair, 11 and 15, hiked to the summit of Ben Nevis in Scotland, Scarfell Pike in England and Snowdon in Wales, along with their mum Milly and cousin Simeon Hayns, 15, from Oxford.
They completed the 24-hour challenge with 15 minutes to spare after scaling a total of 3,408m – about 13 hours of walking.
Milly said: “We did it, national three peaks in 23 hours and 45 minutes.
“The most exhausting but exhilarating experience of my life, so proud of the boys.”
Their dad, Clay, had been on driving duty and ferried the troop from one mountain to the next – a total of 11 hours on the road.
“A few dramas along the way,” he said, “particularly night climbing Scafell Pike in the mist.”
Simeon, Silas and Isaac committed to take on the adventure to raise money for their grandfather’s charity, Zimbabwe A National Emergency (ZANE).
They have smashed their £3,000 target with more than £3,700 of donations, 105 per cent of their goal.
Tom Benyon is the founder of ZANE and to get his ten grandchildren more involved in the initiative he set up a branch of his charity called Zane Kids.
The wing of the organisation focuses on the children in Zimbabwe and money raised will go towards a school and a club foot programme.
Tom created ZANE 14 years ago and the charity has raised £20 million in that time for some of the most desperate people in Zimbabwe.
Along with his wife Jane, Tom walks around 400 miles every year for the charity and the couple are both in their seventies.