It’s way out of my knowledge but it looks like the electrode for the touch is missing. It should be on the left corner where the “02” is printed on.
As you guessed the black thing is a transistor and should switch a led or probably a 5V relay. But please be careful since I’m guessing too xD







Never touched HK but a buddy of mine uses it only. AFAIK HK compatibility is expensive compared to cheap HA integrations. Even hardware not specifically for HA can be integrated like Tuya products with “local Tuya” so the availability is huge compared to HK. Getting into HA takes a bit time but it’s worth it.
There is an HA app on android and iOS where you can create widgets for all kind off stuff ranging from switches over sensors to scenes and automations. Or you can create multiple dashboards for each kid with their own layout.
It’s at least worth a try.