We celebrate the Nativity of the Theotokos on September 8, but begin, as is the Church's tradition, with a versperal liturgy on the evening before. In addition to implementing Jen Marie's great idea of making a birthday cake for Mama Mary, this year I'll be using the Nativity of the Theotokos icon coloring page from the OCA website to help my children understand and commemorate the feast.
To download the full-size icon coloring page (pdf), go here and scroll down a bit. (I am surprised that this icon is not included in the "Orthodox Christian Icon Coloring Book," as it represents one of the 12 great feasts of our Church.)
See Kelly's previous posts for the Scripture readings, links to articles, and other helpful ideas.