It’s summer now.
The water must be at least 16 degrees C. So not Hawaai, but not quite freezing either.
Children chill more quickly, and the loss of core heat is the enemy in the water--still not as formidable as the one on land.
An hour (or less for smaller children) in the water, and the kids would be shivering uncontrollably--very hard staying afloat. If the shooter had (and was capable with) a rifle/submachine gun, it would be very difficult to get far enough offshore to avoid being shot while coming up for air, and then difficult to stay out there long enough to escape.
When we were Cub Scouts, we learned to surface and submerge silently, but that is training few children ever get.
Regardless of the parents' political views, this is a horrible event. My condolences to you and all of Norway.