According to the 1820 census François was age 45 or over as was his wife. Living with them were: 2 males under 10; 3 males between 10 and 16; and 1 female between 10 and 16. From this limited information it is impossible to say if your "Frank" is among the children. Do you have the birth dates for the children?
I noticed the following in the Drouin marriages which may connect: St. Regis, 15 February 1847. Joseph Tecier, son of Joseph Tecier and Marie Robidoux & Therese Isiameie?? (very blurred), daughter of Jacques Isiameie & Catherine Kensainski.
Have you looked at any church records from the Ft. Covington area? I am not sure where St. Lawrence is - can you help me out?