When kiddush is made in shul on Friday night and the wine is given to a child to drink, how much is required to drink? Shulchan aruch seems to suggest just a bit, but then how would the kiddush have worked in those days when this was necessary if a full reviiis wasn’t drunk ?
The amount that one has to drink after saying Kiddush is “kmaley lugmav” (a cheek full, of one cheek, which is most of a reviis, depending on the person. Drinking a reviis is only an extra, but it is not needed. In that sense you are right that the child, or at least one f them should drink maley lugmav. B’dieved even if the maley lugmav was divided one is still yotza, however this shoud not be relied upon unless necessary.
O:CH 271-13, 14, Eliyahu Rabba ibid 18, Kaf Hachayim 271- 83, Shmiras Shabbos Khilchoso 48-14, 49-9.