For balance I believe they tell the truth in the vast majority of cases.
However I can think of at least two cases I knew of where the child had been put up to the lies (one by by a jw mother trying to get a divorce on "scriptural" terms and another non jw divorce case where the mother tried to threaten this to get a better financial settlement.)
Before I get an onslaught I firmly believe that the vast majority of cases are true and that whatever they should always be put to the authorities (social services/police/medical professionals) for handling.