Who are the "except some children of his people (Fir'awn)"... Was the magicians who became muslims one of them?

The word Zuriyyath means children as well as a small number.

So the Ayath could mean that just a few people from Musa's (AS) nation excepted Islam.

If we take the meaning of offspring, then there are three opinions.

The first is that this refers to the children of the people whom Musa (AS) was sent to. The nation of Musa (AS) did not accept his message and died on kufur. Then their progeny accepted the Prophet's message.

The second opinion is that it refers to a few people from Firawon's family who accepted Islam namely his wife, her servant, Firawon's treasurer his wife, and the person the Quraan refers to as Mumin Al Firawon.

The third opinion is that Zuriyyath refers to a group of people whose fathers were from Firawon's nation and whose mothers were from Banee Israeel (nation of Musa (AS)

Another opinion is that the word Aamana here does not just refer to having faith. It actually means to openly declare and attest to one's faith. This has to do with the begining of Musa's (AS) invitation of Firawon towards Islam. A group of youngsters (Zuriyyath) openly testified to the kalimath even though they were not being addressed by Musa (AS) . Then later on when Musa (AS) concentrated on his people, the rest of the nation as well accepted Islam.

I did not find any mention of magicians who accepted Islam under the commentary of this ayath. (Tafseer Qurtubee, Tafseer At tahreer wa Tanweer, Tafseer Tabree)

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai