The simplest way is to add a second motion sensor in the adjunct room and then treat them both as one zone.
Alternatively, there may be an option using core. You can ask in the Core peer assistance thread for suggestions.
As for the pets, probably not in this particular case. Normally to reduce triggers from pets you mount the motion sensor higher or block off part of the sensing field. But neither of those is likely to work in a bathroom situation.
There are motion sensors that work with SmartThings, in particular the ecolink “pet Immune” sensor which can be dialed down in sensitivity so that they don’t pick up small animals. But that typically means cats, not large dogs. And if you can set them to not pick up large dogs they won’t pick up kids, either. So it can be challenging. If the dogs are small enough, say under 30 pounds, it may be doable, but not with the iris sensor.