If you can find somewhere else to live, PLEASE do so. Mold is a HUGE problem for CFers, especially once we're colonized. Dealing with ABPA is a never ending circus, and seasonal allergies alone are very troublesome.
Forget about walking on egg-shells to keep the in-laws happy. It is your JOB and your UTMOST RESPONSIBILITY to protect your child.
I feel very strongly about this because I once a researcher for pediatric asthma. In one home, we observed mold in almost every room, not to mention plenty of other things that were dangerous for respiratory health. The child we were working with in that home DIED of an acute asthma attack. That tragedy could have been prevented.
Please, don't subject your kid to the conditions you've described.