Is it permissible to teach a child supplications and have them say "Amen" to them, and are these accepted by Allah Almighty, and does Allah accept their intercession on behalf of their parents?

The fundamental principle is that a Muslim should supplicate to their Lord directly, as supplication is an act of worship and submission to Allah. Islamic law has not enjoined seeking supplication from anyone; in fact, it may lead to the corruption of the heart becoming attached to the created being. This act is not recommended because the Prophet, peace and blessings be upon him, did not command it, nor was it part of the guidance of the Companions. As for the child interceding for their parents, this will occur on the Day of Resurrection, where children will intercede for their parents to enter Paradise.