This has been rattling around in the back of my mind for awhile, so I finally poked around a bit and came up with this chart from St. Paul Public Schools:
http://title1.spps.org/uploads/text_correlation_chart.pdfIt doesn't have complete conversion charts, but it does give you an idea of the range. Your private schooled child most likely does not have a Lexile because the school chose not to pay for the additional figure (it's extra). The Lexile is converted from the reading comprehension scaled score (SS on your report).