]in one scene at Longbourn where Kitty says "...and I don't see that Lydia has done anything so very dreadful"? Did Andrew Davies get this detail wrong?
Since we do have the evidence from the text chap 46 in Jane's first letter To Kitty, however, it does not seem so wholly unexpected.
followed by this in the second My father and mother believe the worst, but I cannot think so ill of him...as to my father, I never in my life saw him so affected. Poor Kitty has anger for having concealed their attachment; but as it was a matter of confidence one cannot wonder.
When Lizzy returns with the Gardiners they are met by Jane and proceed up to Mrs Bennet's room and it is not until later in chap 47 In the dining-room they were soon joined by Mary and Kitty, who had been too busily engaged in their separate apartments to make their appearance before. Kitty came from her toilette(!!) and was tolerably calm, except that the loss of her favourite sister, or the anger which she had herself incurred in the business, had given something more of fretfulness than usual to the accents of Kitty.
So since she had not been taught good principles by her parents, had an elder sister who was always willing to think the best of people(notice how Jane tries to defend the behaviour of both her younger sisters), was following where ever her paragon, Lydia, led I think she might have expressed herself along those lines. And been perplexed to have a reaction that strong from a father, whose cynically placid comments she has heard all her life.
She is unable to grasp the extent of Lydia's disgrace and she betrays her immaturity several times in P&P2. I think it is well done, if I dare be so presumptious.