Please let me know your opinion about the following I would really appreciate it
My wife will not be fasting on Tisha B’av. I would like to make Havdalah, have my wife drink a little, and my kid whom is not of age of chinuch even chinuch for brachos drink the rest of the shiur. My reasoning is that as far as the bracha my wife’s minimal drinking will do, and as far as the kavod of kos shel bracha it can be done through my child even though he is not of chinuch (similar to rishonim regarding kos by mila on yom kippur)
The issue of drinking the grape juice is really the same as each Shabbos Chazon, when the adults should not be drinking the wine or grape juice. The Mishna Berura says that it should either be given to a child of age 6 to 9, and if there is no such child there, that the adult should drink it. Therefore your wife should drink a maleh lugma, (1.6 ounces) in order to be yotza havdala, and not your young child.
O:CH 551-10, M:B 551-70, Shmiras Shabbos Khilchoso 62 ftnt.114, Ashrei Hoish O:CH 72-3..