Rashi (13): It was taught here to interrupt between verse 11 about idolatry "va'Ya'aseh ha'Ra b'Einei Hashem" and the death of Elisha (which begins with verse 14).

Radak: It was taught here to put the beginning of Yo'ash's kingship next to that of his son. Just like Yo'ash became king in his father's lifetime, also his son did. Perhaps Yo'ash sensed that Yaravam's brothers would contest him, therefore he seated him on his throne. This is why it says "Yaravam sat on his throne" (verse 13), and not "he ruled in place of him", like it says about other kings. Seder Olam (19) says that he ruled for one year in his father's lifetime.

Malbim (10): It belongs below. It is taught here only to explain Hashem's salvation [that verse 4 mentioned] - in Yo'ash's 37th year