Is my friend sinful if she adds lessons in Christian religion for Christian kindergarten children, in order to achieve justice among parents, knowing that she will add Islamic education lessons for Muslim children?

"Teaching Islamic law to children is a religious responsibility and an important task. What they learn becomes deeply rooted in their hearts and easy for them after puberty. For the best hearts are those most receptive to goodness, and the hearts of children are most deserving of instruction, so that religion may become firmly established within them and they may act upon it. As for teaching false religions, it is the propagation of forbidden falsehood, and it is not permissible for a Muslim to do it. And whoever gives up something for the sake of Allah, Allah will compensate him with something better."