There will be times you don't want to dedicate your phone or your wifes phone to being a baby monitor (which may drain the battery fairly quickly). You also need to prepare for less tech savvy baby sitters like your parents and in the case of teenage babysitters you may not want them on your network.
Here are two potential solutions:
get a basic audio baby monitor as a backup
buy 1 or more cheap android phones (but be mindful of the specs) or tablets to dedicate to baby monitor duty (prepaid phones work for this). There's a security setting on android that lets you "pin" an app so that the user can't close it or switch to anything else without a code.