The gemara (Meg 4) tells us that even women are obligated in mikra megillah because “af hein hayu b’oso ha’nes.” The Yerushalmi (quoted in the Hagahos Maimoni, Hil Megillah perek 1) goes a step further and applies the same sevara to ketanim as well. (Parenthetically, you see from this Yerushalmi that “af hein” applies to those who were in danger and saved, not only to women because Esther caused the miracle, as some Rishonim learn.) R’ Baruch Mordechai Ezrachi asks: why do we need to come on to the din of “af hein” to obligate children in mikra megillah? We have a din of chinuch in kol hatorah kulah!?
You can read Rav Ezrachi’s answer here. Maybe the Yerushalmi is not talking about a din of chinuch at all. This is the same Yerushalmi that is the source for the din in S.A. above which deals with little kids. Perhaps there is some kind of unique din here that obligates all to hear megillah.
Whatever the lomdus and however you explain it, there is undeniably some kind of special connection between Purim and children. The L. Rebbe discusses how so many of the minhagim of the day – banging for Haman, dressing up, etc. – are an attraction for children in particular. Chazal tell us that Mordechai countered Haman’s decree by gathering thousands of children and learning with them and it was in that zechus that the nes happened. Limud haTorah by Jewish children is a powerful, powerful force.