Avoid military phrases, and anything that sounds out of the context of normal everyday conversation ( yes, bird food sounds good) .

It make sense from my experience to have prearranged places where you (or member of family-group) can check for messages and warnings ( and have backup place of course)

Also consider some signs how you or someone yours can see that something is wrong (for example some sign that intruders are in house with you or similar).

Just go with your family trough couple of scenarios (in discussion) and see what kind of call signs, codes, and warning signs you could use.