There is a button on the listing you can use but unless there are lots of reports Airbnb are unlikely to act on your report @Sian34. far better to call them.
If the mother is shown as the host and the listing wasn’t as described , Airbnb says on your booking confirmation you should flag this with the host and Airbnb within 24 hours of arrival. Did you contact them?
Not clear from your post . Is the mother shown as the only host - or is the son the co-host? You say he is under age- so you mean under 18?
Also not clear - are you saying you didn’t know the son and a flat mate where living on the premises? And that the listing said you would just be sharing with the mother?
If this is is the case please leave an honest review and a thumbs down.