Last Night's Viewing: Louis Theroux: Extreme Love, BBC2 - Grandma's House, BBC2

In his new documentary, Louis Theroux told us, he would be exploring "the pleasures and strains of one of the most extraordinary kinds of relationship".

He meant the relationship between parents and children with autism, and his implication that every low was balanced by a high turned out – on the evidence of the film that followed at least – to be something of a kindly piety, the sort of thing one says to keep people's spirits up in hard times. Louis Theroux: Extreme Love wasn't entirely bereft of moments of warmth – several of which derived from the presenter's own awkward kindness to the people he met – but strain was far more in evidence than pleasure.

Paula, mother of autistic twins Marcello and Lucy, wasn't seeing a lot of upside, that was for sure: "I just try and make them happy," she said defeatedly, "I am heartbroken... God forgive me, but I don't get a lot of enjoyment from them." Her husband wasn't any more cheerful: "There's not like that laughter part anymore," he said disconsolately, after Louis asked how the twins had affected his marriage, and then he broke down and had to leave the room. Like other parents here, he's haunted by ghost children, by what might have been had autism not intervened.

The containing cliché for what these parents have to deal with is "challenging behaviour", which can range from tantrums to physical assault or a blank indifference that is often more distressing than either. Theroux is used to hostility from his subjects, but even he seemed a little taken aback to get no social echo from his friendly noises of engagement. He also – rather satisfyingly – got a little rattled and socially anxious himself after Nicky, one of the more able of the autistic people he met, looked him up on the internet and started reading his biography out to the camera crew.

Nicky was a pleasure, a kind of living caricature of anxiety at most times but possessed of a sly wit when he relaxed. When his mother explained that he'd first begun talking after a visit to a church he questioned her interpretation of it as a miracle: "I think it's a disaster at the same time too," he joked. "My mouth got me into a lot of trouble through my life." It had recently done it again, after he'd become agitated and threatened to stab a fellow pupil who'd been teasing him. But despite that setback he was graduating to a more advanced school.

I'm not sure that "the world of autism" (Theroux's phrase) is a place you can visit as if it's a tourist destination. And Theroux's final remarks about the parents' love and courage veered dangerously close to the condescension of a tourist. But there was no questioning his good intentions, or his skill with these difficult children. No questioning the film's success, either, in showing just how difficult they could be. "If he breaks anything in there we'll throw it out," said Joey's mother, leaving him in his bedroom to calm himself down. Not so easy if what gets broken is your heart.

Children can be an anxiety even if they're not autistic, of course. In Grandma's House, Tanya worries almost constantly about her son Simon, whether it's to do with his faltering career or a fallow patch in his sex life. The self-reflexive comedy has got even more so for the second series, with "Amstell" the character responding to reactions to the performance of Amstell the actor: "It's good acting! I'm doing vulnerability... I'm stiff in real life!" he protests, after Tanya questions his suitability for the family comedy he's just sold to the BBC. I'm not convinced that Tanya's ghastly boyfriend, Clive, would have been quite so jocular about Simon's one-night stand with a 16-year-old ("If there's grass on the wicket, let's play cricket," he says, in blokey solidarity). But it's very funny all the same.