Why did mama (Isabella) tell Phil "they escaped safely"?
She did suspect Phil but then after looking for the situation she concluded that
the younger kids didn't know anything. (chapter 35).
So why did she tell Phil everyone has escaped safely? That's because she already admitted her defeat (chapter 37), and with that, she had nothing to lose anymore.

 But ultimately, it's because from the bottom of her heart, she actually loves them and didn't want Phil to worry about them