Advice on mould

Hi,
I’m fairly new to the UK, and have no experience dealing with mould. I have a 4 month old, and we just moved into a new apartment yesterday. I spoke with the letting agency several times about how there should be no mould in the apartment and they kept saying they’ve done an inspection and there is no mould.
Yesterday upon moving in I noticed the wallpaper in the bedroom was coming off in the bottom corner of the wall. I looked behind the wallpaper and noticed mould. Got in touch with the letting agency right away and they said they missed that area during their inspection because the wall was behind a wardrobe.
I’m scared for the well being of my daughter and very angry at the letting agency. They are sending someone tomorrow to fix it.
I’m looking for advice on how I can make sure the problem is dealt with as thoroughly as possible. What should I make sure is done? How can I make sure what they do is the right way to take care of the issue? Thank you!