There is a difference between alerting, and actively engaging, like his dogs were. If the dogs were just alerting, they would have remained stationary and barked. But his dogs weren't doing that. Unless they were trying to catch the stones and were playing? But it didn't look that way to me. And the fact that the OP said now that sometimes the dogs bark and sometimes they don't, also gives support to he fact that his dogs were engaging with those children.