It's the adults who see the differences between them, such as religion or ethnicity.
So at the shared campus of Pirniehall Primary and St David's RC Primary in Edinburgh's West Pilton, it is the most natural thing in the world for the nursery-age children to play together for an hour every morning or afternoon when their teachers have outdoor play on the timetable.
"They'd be looking out on to a shared play space and seeing their neighbours and cousins and other people they know, so automatically they wanted to share," says Lynn Goss, nursery teacher and acting principal teacher at Pirniehall.
Four years ago, when the two schools moved to a shared campus as part of Edinburgh's first phase of PPP (public private partnership) school renewal, there was no outdoor play area. For seven months, the pupils - from nursery up to P7 - had to play inside at break or lunchtime.
As each school has found its feet, settling into new premises and a new shared campus relationship, the staff have been able to develop joint working in curricular and social areas.
The sharing of the outdoor play area between the two nurseries has evolved over the past year, following joint planning between management and teaching staff and support from the education authority's early years'
team. The schools' success is showcased in the Scottish Executive guide to twinning between denominational and non-denominational schools, Building Friendships and Strengthening Communities, which was launched last month to coincide with the First Minister's summit on anti-sectarianism.
Rosemary McMillan, headteacher of St David's, likens the schools'
relationship on the campus to two semi-detached houses. Like anyone moving into a new house, they didn't immediately become bosom buddies with their new neighbours. The relationship grew over time. "We wanted to get our own house in order before talking about how our shared campus was going to work," she says.
The initial move was more difficult for staff at St David's, as the school had no permanent head for almost a year. Mrs McMillan was acting head for that period, making it difficult for her to initiate policy in case someone else was appointed to the job.
Once the planning started, however, there was no looking back.
"Because the staff got on so well, there was a willingness to overcome any obstacles," says Mary Gillespie, headteacher of Pirniehall.
The nursery staff appeared to bond, as in some ways they were isolated from their primary colleagues, having different mealtimes from the rest of the school. Now, they meet at lunchtime and share information and jokes.
One of the biggest issues was having to take responsibility for children who were in the other school's nursery, says Mrs Goss. "People needed reassurance if something happened. You don't know how it feels to take responsibility for someone else's charges. In nursery, action has to be immediate."
This meant that there had to be informal sharing of information about children's needs - behavioural and medical - and which children needed to be handled in a specific way.
"We know a lot more about each other's children now," she adds.
The joint behavioural policy needed no more than a little tweaking, but it was important that the same explanations were given to the children, the same sanctions applied, such as removal from an activity for inappropriate behaviour, and then referral to a member of staff of the child's own nursery.
"It was remarkably easy, although it was the thing that worried people most," says Mrs Goss.
As the play area has filled up with equipment - everything from climbing frames to bikes - the staff have worked on how to share the resources, with the area laid out into zones, each supervised by a staff member.
Joint risk assessments have been undertaken and staff have set up a shared behavioural policy to ensure consistency.
Over the past few months, staff, pupils and parents from both schools have been developing a garden area. When finished, it will contain flower planters, a willow tunnel, native plants, seating for the children, a mini-orchard, and some sheltered areas.
Perhaps more important is the ripple effect: the friendships and sharing started in nursery are filtering up to the older classes as children progress through school.
If the nursery class from one school is late coming outside for any reason, the others go up to their windows, waiting impatiently for them to come out.
The children can choose which play equipment they want to use and which children they want to play with. A lot of the time it may be class-based friendship groups, but by no means always.
Nurseries are non-denominational - they follow a common curriculum - so in theory, a child attending St David's nursery could go into Pirniehall's P1 class. In practice, there is very little crossover. But this is an area where all the children live locally. Cousins, neighbours and friends go to one or other school.
Religious observance at this age goes no further than a cele-bration of Easter or Eid or other religious festivals, and the two nurseries come together for joint celebrations, such as harvest thanksgiving.
The primary schools share a number of events, too: some curricular work, St Andrew's Day assemblies, a communal school song, the girls' football team and social occasions. They also have one business manager, which means that budget management and sharing is much easier.
After the Christmas holiday, one school invites the other's staff in to "first foot" them and share mince pies. Before the move to the shared campus, two joint ceilidhs were held for staff and families on each school's home territory to cement relationships between the two communities.
The dining hall is also shared. This means shared duties of meal supervision, so the children have become more familiar with the staff of the other school.
"I can cut up a Pirniehall sausage just as well as I can cut up a St David's sausage," says Mrs McMillan.