Overnight, an ocean has appeared under our window. Late arrivals with kids in tow are always disorienting, but dark as it’d been — the sort of deep, countryside dark that has shape and mass — I’m pretty sure our hilltop chalet (several hours from the coast) isn’t surrounded by water. And yet there it is, lapping at our balcony: a silvery lake stretching across the valley. Not a mirage — this mountainous part of inland France never gets that head-spinningly hot — but a dazzling optical illusion that causes each of our days in the Tarn legion to begin with a sharp intake of breath followed by a deep, meditative ‘ahhhhh’. Trebas-les-bains is exemplary Tarn Valley terrain. Here, where mountains rise sharply off densely wooded riverbanks, morning temperature inversions cause the valley to be enveloped in a thick mist, which soon burns off to reveal a scattering of stone houses that just about qualifies as a town — and not many tourists. France’s beaches tempt most of its holidaying population, leaving this verdant river valley of farmland and beautiful medieval bastides (fortified towns) to the outdoors-loving Northern Europeans who travel here because no one else does.

“Why is it called Trebas, when it’s so high?” demands French-speaking Mia, as we climb slowly into the mountains on rental bikes the next morning. At points, it’s certainly not tres has (very low), she’s right, but for every up there’s a freewheeling down, roads cutting shady tunnels through woodland, grass verges releasing clouds of mint aroma as our wheels brush past.

From Les Magnolia’s, a hotel in truly pleasant Plaisance, we hire bikes, admiring but not tackling this village piled vertically up the banks of the Ranee River, ivy-clad houses set into the rocks. But from here on, virtually car-free roads make for easy pedaling. We pull over at an apple orchard here, a natural spring there, to refill water bottles, test legs and let the odd tractor pass, whose rumbling approach is audible long before it arrives in these tranquil green hills. At Villeneuve-sur-Tarn, the road skirts the water, butterflies and herons wheeling overhead. The Tarn’s fortified towns, we decide, are at t heir most impressive straddling the water as they do at Villeneuve and neighbouring Ambialet and Brousse-le-Château. Bastions against English invasion, they were built at a time when the Vatican encouraged crusades to rid the region of Cathar heretics.

The Tarn’s most postcard perfect village, Brousse-le-Château, comprises a series of stone houses stacked staircase-like up a narrow gorge, topped with a castle museum. Here, kids can try on medieval armour, learn about a captured princess and peer down through a floor grating into a deep, well like prison cell. “This is eeeeasy,” says Ella, when we exchange peddle for paddle power the next day. Shallow, steadily moving waters make the River Tarn a travelator for kayakers, leaving kids with hands free to trail through its mini rapids. Between villages, the river is delightfully devoid of shops or man-made distractions. The six-mile stretch from Trebas to Ambialet takes us much of the day, with plenty of pauses for picnics on tree-shaded river and lake beaches.

But with days spent kayaking, swimming, biking and — on the crystal-clear lake in the mountain village of Villefranche-de-Panat — powering a pedalo, appetites are not sated by picnic alone. “It’s caveman food!” exclaims Ella, tackling a vast platter of rough-cut charcuterie. We’re dining on the trellis-topped terrace at La Chanterelle, on the outskirts of Trebas. Inside, the scene is more medieval than primeval — hunks of meat braising on an open fire. Like so many of the Tarn’s restaurants, unfussy presentation and hearty portions reign supreme here. Rustic fare it may be, but some of France’s most prized farms furnish the Tarn’s tables. Evidence of this can be heard mooing and bleating in the fields raked along the valley; doe-eyed cows that produce a rosy veal that lures top Italian chefs across the border, and sheep whose milk is made locally into the cheese that becomes a legendary blue in the caves of Roquefort, 60 miles away.

Both Roquefort and Albi (a UNESCO World Heritage Sit e) are easy day trips from Trebas, but we don’t want to break the Tarn’s tranquil spell. Instead, we climb again into the mountains, to Ferme de Peyrouse, a family-run farm where kids can take tractor rides, feed chickens and commune with those doe-eyed cows. Afterwards, in the barn-cum-restaurant, Ella sits transfixed, watching aligot being made. The cheesy, creamy, buttery mash potato mixture is stretched like elastic dough, a metre above a tin bath-size pan, before being served. Ella, whose birdlike appetite is infamous, eats two huge bowls topped with steak hache (premium beef burger). “Mmmm,” she says, rubbing her swollen tummy. “I want to make this at home.” Reliant on the unique potatoes, milk and cheese produced in this verdant valley, aligot is among the many things I think we’ll simply have to come back to the Tarn.