Actually, quite believable. Typical police thinking in a case like this is that a child in the vehicle is a *hostage* to the adult, whoever the adult is.
There is some sense in this, because terrorists frequently use children both as shields, and to prevent a shooting defense *long enough* to say, get a car bomb to its target.
The critical, overriding thing is that the armed police, city and federal, in D.C., have to work in two modes: ordinary crimes and terrorism. And the selector switch between these two modes can change in a moment.
If they thought this woman was just intoxicated, or an ordinary criminal, they might shoot her if she used her car as a weapon, or like a weapon. However, if they think she is acting like a terrorist, it is shoot first and ask questions later.
Initially, they were probably patting themselves on the back for “rescuing” the child before “killing the terrorist”.
No, that leaves out the baby could have been rigged to go boom. This had nothing to do with them thinking she had a bomb but something else. Notice the officials aren’t talking. They can’t come up with a spin the public will buy so all they can do is keep mum and hope something else takes the headlines.