I once heard R. Yisroel Belsky say that children below bar mitzvah who are old enough to be obligated in havdalah but cannot stay up late enough to hear it during the summer must say it on Sunday morning. He said that he frequently has (or had) one of his sons say havdalah on Sunday morning. I also saw that R. Yehoshua Neuwirth, in his small book Chinukh Ha-Banim Le-Mitzvos Ve-Dinei Katan (par. 24), writes similarly.
However, I don't know anyone who has ever done this. When I spoke about it with my rabbi, he disagreed based on a particular approach. I later found this approach in R. Simcha Rabinowitz, Piskei Teshuvos (vol. 3 343:1) in the name of Responsa Kimyan Torah (5:28):
A child who did not hear kiddush on Friday night, one need not teach him to say the words of the night-kiddush during the day -- and the same applies to [a child] who did not hear havdalah on Saturday night, he need not make it up on Sunday -- because the obligation is to teach him to observe mitzvos, and not to make up what he missed.