So make them repeat a year until they get it. Throwing them out of religious instruction would seem to ensure that they never do.
But the discussion is not really about "them" (the children) getting it--it's about their PARENTS 'getting it.'
Having said that, I think your suggestion has a good deal of merit--but evidently was not feasible for the parish in question, for whatever reason.
Again, bear in mind that the priest most likely did NOT divulge all the information he has on the families to the newspaper...