Ignoring the fact that this kind of discrimination is flat out illegal, you are forgetting to consider that the Canadian policy allows males to take parental leave as well, so the excuse for the discrimination isn't even logically consistent.
Personally, having lead teams of people ranging from experienced professionals to interns still in school, I find it extremely hard to imagine a small business that is such a special snowflake that "getting a new employee to the stage where they contribute value is a huge cost". I could get interns to be productive in a day or two. In many fields of work, such as early education, there are standardized accreditations so an employer can basically just pick and choose from a pool of qualified individuals who will be ready-to-work on day 1.
As the other poster said, standard fare in parental leaves is to hire a contractor for a 1 year contract. By the end of the year, worst case is your business grew 0%, you get your full-time employee back and it's business as usual. A more realistic scenario is your business grew by some amount that now allows you to hire the temp person full time.