Even if parents teach their children about God there are still issues. Dreadnought initially said that the prayer used would be the Lord's Prayer. As I asked then, which Lord's Prayer? Do we end before the doxology as is done in the Roman Catholic Church or do we continue with "for thine is the kingdom, and the power..."?
Do we say "debts" or "trespasses" or "sins"? Do we end with "forever" or "forever and ever" (we use the later in the ELCA)? Trust me, there would be parents who would object if the version they use would not be the one the school uses. That doesn't even get into the objections from Muslim, Jewish, Wiccan, etc. parents who also teach their children about god, but not the Christian God.