While I agree that a noxious gas eruption would be very unusual, I could see a potential situation where either the baby or dog was affected more quickly, causing the whole party to stop and try to administer care to that member of the family. Smaller humans’ and animals’ systems are more easily overwhelmed, and if, for example, the baby suddenly showed signs of sickness or becoming floppy, the responsible thing to do would be to stop and try to rouse them.
If a gas was odourless or not of obvious concern, then you might not think to leave an area until it was too late, perhaps.
I don’t know the area well, but I understand there are parts of forest in that state where trees have died from the CO seeping up from below the ground.
Additionally, had there been any old refuse used as landfill at some time in historic decades in the area, this can catch alight as part of the composting process, especially in hot weather, which causes underground fires and small plumes of very toxic smoke.