I was seeing videos from this war, mostly that of the Iran kids... ...I didn't understand the reason Israel was fighting them.

Iran regularly promised to wipe off Israel from the map. Thus, they wanted to kill millions of people (including children).

Wiki Destruction_of_Israel_in_Iranian_policy:

Supreme leader Ali Khamenei described Israel as a "malignant cancerous tumor" that must "be removed and eradicated"

Former President Ayatollah Akbar Hashemi Rafsanjani said that the detonation of a single nuclear weapon inside Israel "will annihilate the entire country"

In 2024, Ali Khamenei told: "The divine promise to eliminate the Zionist entity [Israel] will be fulfilled".

Moreover, Iran never gave up on killing Israelis, including children, through its proxies, terrorist organizations located around Israel.

I didn't understand the reason Israel was fighting them

Iran was working to develop a nuclear weapon. Israel and the United States destroyed the infrastructure.