possibley when writing the earlier part of the novel, Austen just concentrated on Aunt and Uncle Gardiner and didn't think of htem having children. Then she may have just "given tehm soem children"... and forgotten that in the Christmas episode she had't mentioned any... and when the Gardiners take Eliz wtih them to the North, leaving the children with Jane was a reason perhaps to leave Jane behind,(and yes she then forgot they hadn't been mentioned as visiting Longbourn at Christmas), Maybe mrs G might feel happier leaving them with sensible Jane than silly Mrs Bennet. I think it might just be an oversight on Austen's part, but possibly it was an oversight caused by the fact that she wasn't as an author really interested in chidlren very much, and possibly becuase her society didn't make such a fuss of children as modern society does...