Paulcornwall. I understand you monitor the playground with cameras and need to remotely control the wooden gate, probably during playing time. You are basically asking for a "door access" control system. The remote control can be accomplish using RF remote, connected to an electric "door strike" or "dead bolt". Both will need power available at the "solid post". Normally is 12Vdc, connected directly to the lock mechanism, but also providing power to the RF receiver. How far is the wooden gate from where you intend to control? Are you able to provide power to the locking "solid post"? The basic design could include an RF receiver, controlled from battery operated keyfob (same used in alarm systems), a 12Vdc "door strike or dead bolt", power supply. Optional, you could add a push-to-exit weatherproof button and a buzzer.
You mentioned "times of crisis", in which the gate need to be instantly locked. This means the gate need to remain "unlocked" except during the crisis period? The design will be different from the basic design. The question will be on how is going to activate the lock? The door strike and dead bolts are designed with "fail safe" and "fail secure" mechanism. For example, if you need to lock the gate at the crisis time, a fail safe will complete the circuit (close) by applying power, which can be accomplish by pressing a button at the remote. But again, is really necessary to keep the gate open during playing time? That means, someone needs to be aware (you or your daughters) of that situation at all time.