Clara is not pleased with the day's arrangements. "A cathedral!" she humphs, like a cross between Lady Bracknell and Tracy Beaker. "All we ever do is go round cathedrals. Cathedrals are so not what you're supposed to do on holiday."

Clara is not pleased with the day's arrangements. "A cathedral!" she humphs, like a cross between Lady Bracknell and Tracy Beaker. "All we ever do is go round cathedrals. Cathedrals are so not what you're supposed to do on holiday."

Given that we have spent the entire previous day hurling round vomitous rides at a theme park outside Barcelona (tip: chocolates y churros and roller coasters are not a good mix), this seems a little unfair.

"Straighten your shoulders, Miss," I tell her. "This cathedral is different, I promise you." As we exit the Metro, the Smurf's fantasy façade of Gaudi's Sagrada Familia rears above us; a hotch-potch of architectural excrescences no theme park ever dreamed of. As I suspected, it conforms exactly to Clara's idea of a fine building.

"Fantastic," she breathes, gazing up at the great porch, "it's got monkeys and dragons and everything!"

"Yeah," agrees Conor, whistling through his teeth, "Een-cray-eeblay!" This is his only Spanish word, gleaned from extensive advertising of The Incredibles movie around the city. His sister has had great fun trying out various phrases, skipping in triumph when market stallholders understand, or kindly affect to understand, her requests, but Con goes at foreign languages on a need-to-know basis. He learnt how to say "A lemonade, please" in Greek on holiday in Crete two years ago, and, until he blew his holiday money on a bargain DVD of Los Increibles a few days ago, saw no reason to expand on this useful locution. ("I think," he told the baffled shopkeeper, in loud, slow English, "I was put on this earth to buy this DVD.")

I had forgotten, of course, that the whole, if not the only, point of La Sagrada Familia, is going up the million or so steps of its dizzyingly sinuous staircase. So up we toil, nose-to-butt with other tourists, stopping only to grab Conor when he hangs, gargoyle-style out of casements, to peer at the ant-people below.

Once at the top, we gaze down the endless stairwell, which seems to writhe under us. From this perspective, I suggest, it looks as if Gaudi had driven a borehole to the centre of the earth. "Straight to the underworld," agrees Clara with a delicious shiver.

Conor is equally impressed by his Lucifer's eye-view. "Of course, I wouldn't do it, Mum," he assures me, wiping his mouth, "but wouldn't it be fantastic to spit from here?"

Back on terra firma, Gaudi is firmly established as the best architect, ever. Later, in the Park Guell, where there are strange elliptical arcades to explore and ceramic lizards to climb on, we visit Gaudi's former home. "I'm going to have a house exactly like this when I'm big," says Clara.

"OK, I'll build it for you," says Con, with off-hand generosity. "I'm going to be a famous architect, you know."

This is a whole new ambition, and I compliment Con on his grand design. "I know," he says airily. "It's increible, isn't it? Increible but true."

e.dickson@independent.co.uk