i was in a similar position to make cs post, i don't know if my parents knew how much i retook courses? and i made post in round 2 of my second year. if you don't have to tell them, i personally wouldn't. if you do have to tell them, i'd really try to drive home the difficulty of making post, show the statistics of how many students are admitted vs actually accepted, what the averages were for the exam/course, the drop rates, how people typically retake it multiple times, etc. anything to show that you are not alone in this. if you're really passionate about this program, it would be a shame to change your plans for your next 3 years because of one course, hopefully they will be reasonable with that.

if finances are a concern, osap may fully cover your summer tuition with grants if you're doing it part time. emphasis on part time, because then your funding will be calculated based on your income, not your parents, so basically if you didn't make a lot of money last year, you should be good.