The fact that the children of Dasan and Aviram stayed with their father was enough to include them in the sin. The children of Korach did teshuvah and moved away from their father so they were saved. In fact Hashem replies to the entreaties of Moshe.

[Korach 16:24](http://www.chabad.org/library/bible_cdo/aid/9944#v=24)

> "Speak to the congregation saying, 'Withdraw from the dwelling of
> Korah, Dathan and Abiram.'"

This leads to:

[Korach 16:27](http://www.chabad.org/library/bible_cdo/aid/9944#v=27&showrashi=true)

> So they withdrew from around the dwelling of Korah, Dathan, and
> Abiram, and Dathan and Abiram went out standing upright at the
> entrance of their tents together with their wives, their children, and
> their infants.

This implies that the children of Korach did withdraw from around their fathers tent and did teshuvah.