“Play” is an adult word to avoid understanding how complicated, rich and serious children’s experience is. In children’s mental life there is no distinction between play and living.
Dr Franco La Cerla
All too often, architecture for children means special spaces which follow the formulaic and patronising lead of Macdonald’s and Disney: bright colours and blobby cartoons which tell everyone, including children, that these spaces are for them.
In fact, children are acutely sensitive to the subtleties of the real world. New born babies prefer to look up at a complicated canopy of swirling leaves rather than a simplified rotating mobile of plastic cutouts: a crucial instinct in their quest to understand a complex world.
Children are both fanatical about repetition and hungry for surprise. Within a secure space, a space which they can fully own, they chase riffs and variations on a bedrock of highly ritualised expectation.

So:
Make solid buildings with generous spaces which are easily adaptable day to day and year by year to differing needs: spaces which change as children do.
Modern children are prisoners of indoors. Give them views onto nature and onto busy streets, and large spaces full of sunlight, and big windows streaming with raindrops.
Make easily crossed thresholds between indoors and safe outdoor spaces.
Let children explore the real complexity of life, art and architecture. Miniaturise where necessary but don’t simplify.
Focus on the floor: it is the most important play surface for most children and is usually an underused resource.