You will apply for PR for her outland through Buffalo so the processing time is not 18 months, it is 40 days in Mississauga, see
http://www.cic.gc.ca/english/information/times/perm-fc.asp and then up to 11 months in Buffalo, see
http://www.cic.gc.ca/english/information/times/perm/fc-spouses.asp#westThe 11 months is how long it takes to process 80% of cases so your case will likely be much faster because background checks and medicals for a small child will be fast. However, nobody can tell you exactly how long it will take.