My own "rule" of prayer has grown from the prayers provided in the Red Prayer Book published by the Antiochian Archdiocese. Since these prayers can get a bit lengthy as a group event (especially if one of your group is a two-year-old), we typically just say the Trisagion and Lord's Prayer with Noah on the way to school and before bed.
After Pascha, we added the Paschal Troparia to our prayers (with Noah enthusiastically singing along). More recently we've added "Jesus Loves Me." Noah asks to sing these songs as "Trampling" and "Jesus."
The most recent addition to our family "rule" has been the addition of personal commemorations. This started, I feel certain, as a made up song while driving (probably home from church). Noah probably asked me to sing a song about mommy. I responded by singing "I love mommy" three times in the same way that we sing "Lord have mercy" in triplicate as a response in the litanies. At prayer time, this has evolved to be a series of short songs about everyone in our family, as well as their dogs, and sometimes Noah's shoes. It goes something like this:
Noah: Sing mommy
All: I love mommy, I love mommy, I love mommy
Noah: Sing daddy
All: I love daddy, I love daddy, I love daddy
Noah: Sing baby
All: I love Samuel, I love Samuel, I love Samuel
Noah continues racking his brain to remember all of his aunts and grandparents and our dogs. At times he pauses, obviously deep in thought, to try to remember who he has left out.
Noah: Sing Granddad
All: I love granddad...
After he can think of no one else, Noah smiles, very pleased. "Everybody," he exclaims.
All: I love everybody, I love everybody, I love everybody.
I can't think of a more perfect prayer. From the mouth of babes, indeed.