No kidding. Russians shouldn't be surprised.
On the other hand, the kids don't deserve to be attacked by street thugs. Poland is correct, that this is a common crime, and should not be made into a political issue. On a human level, though, someone from the Foreign Ministry should already have gone personally to the families to express concern and regret. I would bet that they did so. If they didn't, they should.
I definitely agree that this issue should not be politicized. The thugs should be punished appropriately and Polish authorities should do their best to prevent such things from happening again. Criticizing Russian leadership is not anti-Russian. However, if it turns out that these thugs really attacked kids because they were Russian, then it should be appropriately condemned. No decent person would beat somebody up simply because of ethnic origin.
Perhaps I'm just having a Lukashenko-paranoia day, but I wouldn't be entirely surprised to find out that, when caught, the "hooligans" might have some connection with Belarus. It is Lukashenko is stand to gain the most from Polish-Russian tensions, not Russia.