How do I monitor voltage on a circuit even when switched off?
The simplest solution is to teach babies to just sleep in a noisy environment. That might be easier said than done though.

As an alternative you could make this which saves some components.







simulate this circuit &ndash; Schematic created using CircuitLab

Note that the indicator lamp should not use too much power as it adds to the power needed by the bell. I would suggest using one or more 12 V LED lights, the ones that replace 12 V halogen lamps for example. These are low power and very bright.