[quote=@TheMadAsshatter]
Right, so I had a talk with my dad about it, and he explained that they were just hugging for, like, 5+ seconds.  He has also said that, even though they both know that my dad is, like, madly in love with her, they haven't done anything line-crossing.  Not sure I trust him entirely, but I'll give him the benefit of the doubt on that.

He also explained that there are other things going on that I wouldn't have counted on.  It's still a bit of a harry situation, but, if my dad is to be trusted in this regard, no actual cheating has occurred.
[/quote]

Then that's honestly all you can do right now.
As much as the cheating can hurt her children, this is something you have limited knowledge on.

On top of which, it's an issue where nothing wrong has been proven.
And even if it get's proven, the way it's exposed could be more devastating to the children than anything.

In other words, the best you can do now is sit back and trust your father again.
Reconsider perhaps once you see something that may confirm it, but for now acting any further wouldn't be wise.