Watch the video. LE had surrounded the vehicle looking in every window from close range. If the child was in a baby seat they surely would have seen it. Perhaps the child was hidden from view.
Of course I have been watching the video repeatedly all day. My 30 years in LE and security operations is primarily what prompted me to add the part about how the possible or even likely knowledge that a child was within would or should have caused the cops to hold their fire.
If the child was hidden from view, why? It adds to my mystification as to why she came to D.C....?