What do you suggest if I tell them by myself that I say to the kids
Either you or your wife will file for divorce (if one of you decides that is the right thing to do). Personally, I would let the mother tell them if she will be filing, or if you have decided you are done with the marriage and will be filing, then you can tell them.
If you decided to tell them first, I would pick your words extremely careful. I like Steve's words above.
I used "It ran it's course." with friends and acquaintances. (Got that from "The Curious Case of Benjamin Button")
The whys will come up. "That's between your mom and I" is a good response.
MWD is against exposing the affair. If you have dug through any of of my quote threads, you may have seen that as well as lots of ideas on "telling the kids". Lots of agreement about age appropriate statements.
"What is best for my kids is best for me" Amor Fati Link to quotes: https://www.divorcebusting.com/forums/ubbthreads.php?ubb=showflat&Number=2879712
