This kind of depends on what the structure of your project actually is. So far your tests all seem concerned with the operation of the system as a whole, i.e. You appear to be doing only
black box testing.
If you want to test the units independently then you know need to open up the system and look inside it (
white box testing). In this case you have stated that there is a sender and a receiver, but you haven't elaborated on how they communicate. If you wish to test either unit in isolation then you need to replace the other unit with something else. In software testing this is commonly a mock object that responds as the real component unit under a limited set of circumstances(although different mocks can be used in different tests so that we fully test all the functionality). In hardware you might want to use a reference device, one that is known to behave correctly in those circumstances, or you may simply wish to use a test rig which performs a similar role.
Also you can further break down your system into its components and test those. So for example, you might want to test how far the unit could be away from the child, so you test the microphone and trigger that detects sound as a unit without the components that send the signal to the receiver. Later you might want to test the range that the two units can be apart, so you could fake inputs from the alarm trigger and simply test the send and receive components.
Now since this question was asked in SQA, I assume (since this isn't Electrical Engineering) that you are really building a prototype out of two PCs linked over a network, and a lot of the hardware discussion above might not seem relevant. The point is that you want to approach this on multiple levels. Consider the system as a whole, and consider it broken down. Look for points you can separate and test the components that come out of that structure.