But blabbing it to the press is
There is a saying: Three can keep a secret if two of them are dead. Once a few people know about something, it's going to come out. If Shriver felt that it was better to control the release after letting her kids know, who is to say she is wrong?
Neither Shriver nor the children caused this mess and none of them have an obligation to pretend that Schwarzenegger is a good father, a good husband or a good person.
On top of all that, Schwarzenegger and Shriver have four kids: the oldest is 21, the youngest in his teens You make a huge assumption that they didn't know or suspect something was going one when their dad was spending so much time with this woman and her child.
Put the blame where it belongs: on the cheaters, the adulterers and the liars, Schwarzenegger & the other woman.