Breaking out of loop (graphical programming)

That is way cool!

I would start just with the door HES and the scale.

To get the belt sensed may turn out difficult, and a changing of the buckle is a legal/insurance thing (may be the buckle has already something for such a sensing but it is not populated...). For sensing that the belt is unbuckled, you could just take a small box where the belt tab has to be placed/parked. An if it is not parked, it considers it buckled.

Next thing is the scale: did you check if the scale stays put? Most scales have a timeout... in other words, you may need to switch the scale on and off with Espruino to make sure it is on. The issue then with most scales is, that they try to find their 0 (zero), and if the weight is not stable, it goes to error. if you can find a capacitive sensor, that could help. Said that, you may just use a device where the child seat belt tab has to be parked in order to consider a child in or out, and then deal with the door. In todays cars with airbags, the driver and the front passenger seat have already an even weight distinguishing senser. Check out what these sensors are and you may go the same route for that. You put such a sensor into the child seat (under the lining, between the fabric (with padding) - which you usualy can take of for washing).

Other options are sensing weather the key is in the ignition (and partially turned...) just came switched on of just switchet off, etc... that's easy: most cars have a outlet (cigaret lighter) socket that disconnect from 12 V power as soon as the key is taken out (to prevent the battery going down). Plugging your device into that gives you the signal right away: when power switches from external 12 volt to your device powering battery... Using that 12 socket is at the same time a charging source... ;-) - you get me going here...