The Gemora explains the Mishna that after a man counted his children on the Korban Pesach, he told them that only the first one to get to Yerushalayim can partake. However, he only said so to push them to come faster. The Mishna tells us that all may partake.
What is the Chiddush of this Mishna?
Although the man originally counted all of them on his Peach, it would seem that he removed them from the Pesach – he said that only the one who gets to Yerushalayim first may partake. Why then can they eat from the Pesach?
Rav Chaim Solovatchik explains that this Sugya is following the opinion that שה לבית is not Mi`d`Oraysoh and therefore the way that the father can count his young children is through the זכיה (Zechiya) facility. However, to remove them, which is a Chov he cannot.