Every child is an artist. That's what Pablo Picasso said.
Not every child, though, is an equal partner in a professional art collaboration with their mum and dad.
As well as being the bundles of innocent creativity that Picasso probably had in mind, Merrick, nine, and 12-year-old sister Hope have been involved in their parents' performance art since before they knew that's what it was.
When Merrick was one, he starred in his mum's tender 12-minute video I Love My Baby And My Baby Loves Me.
Later, Grace would take the children along as she devised her performance routines. "They were watching us make our work and they were around when we were in rehearsals. They were playing in a studio while we were playing in a studio." So it seemed natural to play together.
Grace choreographed a short performance duet for herself and Hope, and another for Gary and Merrick. The latter, titled Would You Rather Be Lost, went on tour - and looks like just about the most fun a father and son can have without the involvement of water slides or candy floss.
Those projects went so well that, a year ago, Grace and Gary decided they and the children should work together as a family.
They have now made two films for the National Trust to mark the 200th anniversary of the Peterloo massacre, when the cavalry charged a workers' rights protest in Manchester, killing around a dozen people.
Artists Jeremy Deller and Bob and Roberta Smith and singer Jarvis Cocker are among the others taking part in the National Trust's People's Landscapes project. One of the family's films will be shown at Quarry Bank, a historic mill in Cheshire, where the family have filmed around 100 local schoolchildren drumming around the site.
Doing interviews about their creations is not nearly as interesting as making them, and at Quarry Bank, Merrick gets bored and disappears from the cafe table less than three minutes after the family start telling me about their work. He's bursting with energy. He's a nine-year-old boy.
Hope is more patient but she too makes her escape before the interview is over, boarding a National Trust buggy to the next filming location where some steel drums are waiting. Merrick reappears from around the corner at one point and his mum suggests I ask him a question before he bolts again.
Do the family all come up with the ideas together? "Yeah," he replies. He pauses, then reconsiders. "Not really."
"We have," his dad protests.
Are there any bits that were your idea?
"No," Merrick says - before his mum and dad both remind him about the bits he suggested.
"The drumming was your idea," Grace insists in a light-hearted rebuke. "The whole thing was your idea!"
It has been a long day, and the fact they are artistic collaborators doesn't take away the fact they are parents and children first and foremost.
Rolling in mud
In their Quarry Bank film, the sound of the young drummers, who play bins and buckets and anything else that will make a beat, represent the sound of protest and marching and militaristic rhythm, Gary explains.
Gary reminds Merrick too about rolling around in the mud at Dunham Massey, the other National Trust property where they have been filming. There, they dressed in period costume to present themselves as a sort of echo of the family who lived on the grand Cheshire estate in 1819. Their film project is titled Glorious Phantoms.
"We've sort of contributed all together," Hope picks up. "In Dunham Massey, I'm doing a speech all on my own, which is the Greta Thunberg speech to the UK Parliament."
The march at St Peter's Fields in Manchester in 1819 was about political reform at the dawn of the Industrial Revolution. The family tried to think about what was the biggest issue inspiring protests today.
Grace says: "Hope has seen and heard of Greta, and been involved in climate strikes at school, and you were saying, weren't you, that it feels like the most important thing that you could think of right now. At the time of Peterloo, the important thing was to have a voice, to be heard. And Greta's like a spearhead for that [today]."
As well as the films, the family, who live in Ilkley, West Yorkshire, are also working on a new stage performance and have been participating in a project for Manchester theatre company Quarantine.
That has involved them staying in a new housing development and organising creative activities for the neighbours to encourage a community spirit, including painting workshops, country dancing and ice sculpture.
It's a great excuse to have the kind of quality time most families don't get, but they have also made a point of discussing why they are doing those things in the first place.
'It may stop at any minute'
"You have to find a way to talk about the work, about what an artist is, what art is," says Gary, who is also one half of the performance duo Lone Twin. "It comes down to those fundamentals sometimes - about why things happen, or why an idea might be good."
For their work for the National Trust, the children are getting paid for their contributions, as any professional artists would. It's a good source of pocket money as well as being fun, but Grace says she and Gary know the arrangement might not last.
"As soon as they go, 'This is boring, and I don't like it, I'm not getting anything from it, and why am I doing it?' then it will just all be packed away. Gone. It's not about making them do something. It does work now but it may well stop at any minute and we have to be prepared for that."
Lots of artists who work together have their creative differences, after all. But these collaborators have to live under the same roof, and being a family comes first.