The terrorists probably didn’t want to get slowed down by killing the receptionist, because they wanted to get two targets in particular, the cartoonist and the editor, and they needed to identify them. They killed them both and then shot all the other people, including the police outside. The woman and her daughter were hiding under a desk at that point and they probably couldn’t see them and didn’t want to take the time to look for them, because they weren’t important.
The key here is the plan seems to have depended on a silent entry. They didn’t blast their way in, they needed to get in quietly and get upstairs to the editorial office.
If the lady had not put in the code, if they had started shooting on the street, it would have been a very different situation.
Yes, in all likelihood the lady and her child would have died at the door, still locked, the terrorists still on the street.
Israeli security guards commonly face a similar situation when they try to tackle a bomber before the bomber can get into a supermarket, for example.
ANYBODY who worked at a known terror target like Charlie Hebdo, especially those who knew the entry code, should have been better prepared for exactly this situation.