If a baby tooth has a cavity, does it have to get a filling?

Generally we would recommend yes, but there can be exceptions.  Any tooth with a cavity is full of the bacteria that cause cavities (makes sense).  The decay in the tooth acts as a reservoir to seed the cavity-causing bacteria to other teeth, possibly causing the dental version of a decay wildfire.  In cases where the cavity is limited and the baby tooth will be coming out soon, we (dentist and parent) may opt to not place a filling and wait for the tooth to come out.