In the same complex where an entire colony of people hiding behind barricades were already overrun, yes, it was a bad idea. As the movie eventually showed, xenomorphs are not restricted to using specific hallways and means of entry. They can be in the ceiling, underwater, under the floor, in the air vents, etc.
If something unexpected happened, Newt could've been taken, killed or they would at least need to go retrieve the child before they could evacuate. That would cost valuable time, and possibly get people killed unnecessarily.