She knew he'd eventually have him killed...probably sooner than later. In the scene she visits to plead with Ciccio to spare her only surviving child. She probably knows he will refuse which is why she brings the knife and ends up sacrificing her life for that of her son.

I would suppose that she brings him along to ensure his safety; since she knows men are determined to kill him, she would not want to let him out of her sight for a moment.

In the Saga there is a deleted scene where 2 of Ciccio's men come to their home to get Vito. Mother defiantely says she will take him to Don Ciccio herself. Perhaps if this scene had been left in the film it would've made her intent easier to understand. (Although in my opinion, the dialogue itself make that pretty clear.)

Apple