Well, here's the HTA:
http://www.e-laws.gov.on.ca/DBLaws/Statutes/English/90h08_e.htm
A quick search of "child" doesn't seem to indicate any law prescribing age in regards to front or back seats. The MTO guide I linked above says:
"Once your child can sit all the way against the vehicle seat back with legs bent comfortably over the edge of the seat, and with the shoulder belt flat across the shoulder and chest, he or she is ready to move from the booster seat to the vehicle seatbelt."
It seems there's no law, one way or the other, regarding front or back, so I'd have to guess she'd be legally allowed to sit in the front seat once she's out of the booster seat.