Mammals, Playhouse, Oxford <!-- none onestar twostar fourstar fivestar -->

Given their persistent presence in most middle-class households, it's odd how rarely nits crop up in domestic stage comedy. Children, too, don't get much of a look-in because of the administrative difficulties in using young performers.

Both nippers and nits come into their own, however, in Mammals, Amelia Bullmore's engaging, perceptive and often sharply funny play about a marriage in crisis. Here, the tots - Jess, six, and Betty, four - are played by adult actors (Jane Hazlegrove and Helena Lymbery), a device that amusingly illustrates the physical and mental space children take up in our lives. Rampaging round the toy-strewn kitchen, or asking awkward questions about death, sex and "hairy fannies", these not-so-little brutes forcefully bring home why their mother Jane (Niamh Cusack, excellent) feels so overwhelmed when she's holding the fort alone.

The marital turmoil begins when her husband Kev (Daniel Ryan) returns from a business trip and says he's fallen in love with a colleague. The couple barely have time to draw breath, let alone consider the implications of this bombshell, before they are invaded by Kev's best friend, Phil, a witty, popular underachiever (attractively played by Mark Bonnar) and Lorna (Anna Chancellor), the latest angular, high-maintenance girlfriend.

Their arrival triggers an overplotted series of confessions that raise the question of whether "rubbing along" together is enough. Is fidelity a true compliment to a partner, or just a sign that the heart has gone dead? Is what we think of as steady love just a holding operation, or a form of self-deception practised in the interests of a quiet life?

The territory may be familiar, but Bullmore finds fresh angles. I liked the way the marital rows have to be tucked into the few gaps left by the routine of parental drudgery. "This is my house. You only live here," booms one of the kids when her often absentee dad refuses to tell her what he and Jane need to talk about.

The mutual blackmail between parents and children; the casual betrayals (Jane shopped for resorting to slaps, even though she's tried to silence them by alleging that "Daddy would be really upset if he knew you made me cross"); the male double-standards that leave Kev feeling both relieved and peeved that Phil did not respond to Jane's sexual overtures - Bullmore observes all these with an undeceived, uncynical eye.

The ending relies too much on a late contrivance, but Anna Mackmin's direction expertly responds to the blend of uncomfortable truth and sitcom buoyancy, and the clever device of the outsize kids lifts the show out of standard TV naturalism.

And the nits? Well, the child who hasn't got them is jealous of the one who has. Parenthood: it's a head-scratcher.

Ends tomorrow (01865 305305), then touring (see www.bushtheatre.co.uk)