What should a Muslim girl do who has recently converted to Islam, and her non-Muslim parents prevent her from praying and compel her to stay with them, which sometimes forces her to pray in the bathroom for fear of missing the prayer time?

Prayer is a great pillar that is not permissible to abandon, and a Muslim prays according to their ability. If informing the family would lead to an apparent harm, then it is permissible to combine the two prayers (Dhuhr with Asr, and Maghrib with Isha) either by bringing them forward or delaying them, to remove hardship and difficulty. As for praying in the bathroom, it is forbidden, because it is a dwelling for devils and a place for uncovering private parts, except in cases of extreme necessity, such as the presence of severe harm and the unavailability of any other place for prayer, even when combining prayers.