I am sure the intent was to save points and not leave the child in Y. However there's another issue quite apart from the fact that 3 is far too old (and big!) to be a lap child ... that of catering etc. If the girl is "sitting with" her parents, but there are only 2 J class seats.. what.. "oh just get her a kids meal from Y" or something?
I agree the best solution here is for all 3 to have seats in the same cabin, most probably PE (3 middle seats ideal - adult on each aisle and the little one in the middle).
Even in J, specially the newer suites, it's ot a great setiup for 3 specially if one of them is a youngster in their own suite.
my 2 cents.