It is objectively reasonable for a witness to assume that an abduction is taking place when someone comes up to a woman and forcibly takes her child as she starts screaming! Many people would likely have dogpiled/tackled the likely offender and kicked the snot out of him before the cops got there. The fact that the OP and his associate only followed the offender while talking to the police on the phone shows great restraint - and greater wisdom and due care - than many others might have exhibited.
The scenario as the OP describes would give most any reasonable person good cause to act. Once again, kudos!