פירוש על חגיגה 3:2

1. Which minor etc. - but from then and on (i.e after he is after to ride on his father's shoulders etc.) even though he is not obligated from the Torah, the Sages placed on his father and his mother to train him in commandments.

2. Three times (regalim) - one who is fit to ascend on his feet (regel) [Translator: Regel means time but also feet in Biblical Hebrew] the verse obligates, and since that an adult is exempt from the Torah (who is unable to walk themselves), (similarly) a minor is not subject to training (in the same scenario).