Probably the smell alone is not dangerous, but what it implies is too many cats in too small of a space, which can be dangerous and dirty for other reasons, such as cat-box exposure and fleas. Is the odor in your home or her mother's home? If it is in your own, I would take some strong steps right away to remove the overload of cats and to clean. If in her mother's, it will all depend on what impact you can have.
It's not the odor that affects us...it's the urine and the litter that does. I had a cat (whose potty box was kept in my room) while pregnant and still have her. I never let her box get bad though. It's scooped daily and changed weekly. If you are worried talk with your daughter/son about it. Express your concerns and perhaps they can scoop and change the box more frequently. I do know that there is an all natural litter (it's expensive...but worth it) called Scwheat. It's made from wheat and other natural ingredients and is amazing. It really helps with odor and does not have the toxins regular litter (such as clay litters, tidy cats, etc) has which are activated when kitty goes pee. Once my boys turned 2 we switched back to tidy cats (we have also moved since then and her box is now in a more...private area...for her use and I'm the only one who comes in contact with it, besides her) and haven't had an issue.
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