The children shown in the photo are unicycling to a primary school here in Assen, unattended by any adults.
They're riding on the sidewalk alongside a bicycle path which provides the direct route to this school. It's perfectly safe to let children do this sort of thing here.
There is another view of the same school, including a very short glimpse of a child with a unicycle, in my now quite famous video of children riding to school.
Lots of children here seem to know how to ride unicycles and it's not all that uncommon to see them going somewhere on them.
It doesn't even seem to be a particularly recent thing. This video (from BeeldenGeluid) shows a child unicycling to school in 1956:
The presenter says how "there are 500000 normal cyclists in Amsterdam, and one on a unicycle. He is joined by a peleton of other cyclists on the way to school. He is without bell, brakes, lights, steering but the police can't do anything about it."
This rider was 16 when the film was made, so he'll be 69 this year. I'd like to think he is still be riding that unicycle.
Note how he rides through something I've commented on before: an older Amsterdam which lacks cycle paths. It is sometimes assumed by people from other countries that the Netherlands has always been as it is now, but that is not true. There is a continual process of improvements for cyclists.