Mold and mildew need moisture to 'exist'. That's why, as you already know, that bathrooms are notorious for such 'things'.
You might want to look at other 'things' that contribute to adding moisture to the house, thus, leading to possible activity of mold and mildew.
- Damp/Wet basement or crawlspace, - Humidifier set too high, - A lack of a DE-humidifier, and - Cooking w/o exhaust vents to rid the kitchen of moisture.
I'm sure there are other contributors but they escape me at the moment. As for test kits, visit My Favorite WEB Search Engine and start reading. There are LOTS of them out there. Do as much reading as possible to see what best fits your 'situation'.
Yes, it's possible to have 'hidden' mold and mildew problems but it would need moisture/dampness to 'keep it going'. You may have damage behind walls from PAST mold and mildew but after a while, I believe it's not a factor. Remember, it needs to be 'fed'.
My best to ya and hope this helps.
Jay J -Moderator
PS: Talk to your Pediatrician too to see if the kids can be tested for allergies in this area ...