‘Yes, absolutely,’ I reply, anxious to get out before either a full-scale war erupts between the two mothers, or I have to admit to a now indignant parent that I haven’t been entirely truthful. ‘Do you need the toilet before we go, Isaac, or can you wait until we get to McDonald’s?’

‘I need to go now,’ he announces. ‘Will can take me.’

‘Umm, no,’ I tell him. ‘Auntie Tilly will take you, because that’s what we told Mummy we’d do, OK?’

‘I want Will to take me.’ Isaac looks mutinous.

‘I tell you what,’ Will says, sinking down so he’s at the same level as Isaac. ‘Why don’t you go to the toilet with Auntie Tilly, and I’ll get our shoes so we’re ready to go when you come back?’

Isaac considers this for a moment. ‘You’ll be here? Promise?’

‘I’ll be right here,’ Will assures him.

‘Thank you,’ I mouth at him as I take Isaac’s hand. Will may be brilliant with him but I’m not sure how happy Tash would be about him taking care of Isaac’s personal needs. When we come back, Will is exactly where he said he would be, but is looking confused. ‘Everything all right?’ I ask him.

‘Fine. I just had some woman come up to me and tell me what a great dad I was. She said she’d been chatting to you earlier. Do you know anything about that?’

I can feel the blush spreading across my cheeks. ‘She made an assumption and I, umm, couldn’t be bothered to correct her,’ I explain. ‘What did you say?’

He smiles. ‘If I’m really honest, I couldn’t be bothered to correct her either. I just said you were such a great mum that I felt I needed to step up.’

‘God, we’re the very definition of smug parents, aren’t we?’ I tell him with a laugh. ‘We’d better get out of here before we dazzle them any more with our brilliance.’

* * *

We may be brilliant fake parents but, by the time we get back to Tash and Greg’s late that afternoon, Isaac is definitely flagging and so are we. As well as lunch in McDonald’s, we’ve pushed him on the swings, taught him to play Poohsticks on one of the bridges over the river, and debated the merits of pretty much every character in Thomas the Tank Engine while walking round the park.

‘Hello, darling,’ Tash says as she opens the door and he folds himself into her. ‘Have you had a nice day?’

‘The best,’ Isaac tells her excitedly, somehow finding a second lease of life. ‘I went on the big slide with Will and I had a Big Mac for lunch. Why didn’t you tell me I liked Big Macs, Mummy?’