Your justification to use force is limited. You may use force while attempting to make a citizen's arrest for a felony you witnessed, but there is nothing in this story that would imply the onlookers witnessed the crime. You may use force when there is an immediate threat of great bodily injury or death to you or someone else, but there is no such threat implied here.
So are you really prepared to draw down on someone a child identifies as a molester? What if the child mistakenly identifies someone else as their attacker? What if someone reports you as the aggressor in this confrontation?
The most reasonable thing to do here, is to immediately report it to the police and be a good witness. Do not impose yourself into circumstances for which you do not have all the facts. Assert only the level of force you can reasonably justify- threat of death, great bodily injury or in limited circumstances- an citizen's arrest for a felony commited in your presence.