Can’t she prayer silently?
I would assume she was praying out loud in order to teach her children.
Sure ... she can pray silently in the secrecy of her home. She is Catholic, though, and was denied the sacraments of her faith.
Have you ever seen a video of what happened in Russia and the Ukraine under communist rule? I have watched several; they are all heartbreaking. In one video, you see people running out of the churches clutching crucifixes, icons, prayer books while the soviets use cranes and wrecking balls to tear down the physical structure. These believers went underground. From time to time, a priest would visit, traveling under the darkness of night. Meanwhile, the christians would gather in the house of a believer, scheduling their arrivals in large gaps so as not to alert other villagers. The homes often had a hidden room where they would hide the priest.
Recently I watched another video. This one showed a priest sent to re-establish the christian community in 1991, following the fall of the Berlin Wall. The area to which he was assigned was equal in size to Portugal, Spain, France and Germany combined. He was the ONLY priest! When he needed to go to confession, he had to board a plane and travel to Moscow. As he began to rebuild the community, he asked the children to assist him with the prayer books that were written in a liturgical form of the language with which he was unfamiliar. It took him two years to learn that language after which he realized that not even the children were familiar with it and had given him nonsense words to say. He met an old woman who explained how when the KGB came to their village, they went to the school and asked the children: "which of you pray with mama or papa at home?" Her two children raised their hands. They were taken to a re-education facility and the woman was arrested at her job. She was sent to a concentration camp. After several months, a KGB officer took her to a large field, handed her a shovel and told her to dig her grave. When she had finished, he used the butt of his rifle to shove her into the grave, pointed the rifle at her and asked: "Your God is or is not" (meaning is your God real?) She realized that if she said "My God is.", she would never see her children again. If she said, "My God is not.", she would never see God (Mat 10:33). She responded: "My God is." She was very surprised when the officer did not shoot her. Several months later, she was released from the camp and reunited with her children. After that, she never spoke about faith again with her children.
You can get a sense of what life was like from this video. In it, a young girl speaks about the persecution suffered by her great grandparents during the communist years.