As stated in the comment currently on Saswell TRV the child lock setting actually states if device can be child locked or not. To actually enable a child lock press once to enable a display, device must be on (displaying temperature), then press and hold for about 5 seconds - device will display LC - child lock has been enabled. Disable the same way, or by turning off the child lock setting.
Tested on Saswell SEA802 (clone - HAMA _TZE200_yw7cahqs)
https://github.com/Koenkk/zigbee-herdsman-converters/blob/87cbe9fab80628476a7204459d5e0a831b612624/converters/toZigbee.js#L5301
As stated in the comment currently on Saswell TRV the child lock setting actually states if device can be child locked or not. To actually enable a child lock press once to enable a display, device must be on (displaying temperature), then press and hold for about 5 seconds - device will display LC - child lock has been enabled. Disable the same way, or by turning off the child lock setting.
Tested on Saswell SEA802 (clone - HAMA _TZE200_yw7cahqs)
From the user manual: