First of all, this is not a truism that should be used as a guide to protests. The police have been known to mass arrests peaceful marches. In general, yes, large marches and rallies are pretty safe for children.
There are several reasons why the police attacked protesters in Oakland last week. One reason was a show of force to make up for looking bad during the shut down of the Bay Area several weeks ago. More importantly, the police are engaged in pre-emptive violence against what they've concluded are the hard core of the anti-war movements. Their violence is in part an assertion of authority and in part an effort to divide the anti-war movement.
The police *are* engaged in campaigns to divide protesters and they've discovered that activists can be counted on to divide themselves over tactics. The police violence is designed to marginalize those who advocate direct action. It remains to be seen if the more liberal activists can bite their tongue and not further the interests of the police by blaming more radical protesters for the police violence.
Chuck0