Most Helpful Guy
Tell him. Your wife betraying your trust is one thing, but your wife and your child betraying your trust is worse.2
Most Helpful Girl
Talk to mom about What you saw... although it may be Not Everything you Think it could be with Not "Cheating" but Instead "Cheating" dad at the moment with Simple------Sending love messages.
You need to sit her down and have a long serious chat with her. However, in a sticky situation like this one, you don't want to be the monkey in the middle, especially at your tender age. Stay neutral, it's your mom and dad, whom love you very much. Don't take sides, you will not get anywhere and in the end, until you are old enough to fly the coop, just accept it for what it is, no matter how hard here, dear.
This kind of thing, although may be just a Fling and something simple that mom is going through right now, happens in every household. Don't feel that it is only happening to you. Just be there for both of them, should it ever come down where matters become more serious and precarious other plans are made that involve you, their little bird in the love nest.
Good luck. xx0