Ah I see, your second post contained particular details that you did not include in your first post, so that changes the situation for you and your child.
Yes, this was known as
5(2)(b) of the 1977 Act, which was a special grant of citizenship for those born from a wedded Canadian mother or an unwedded Canadian father before 1977. Based on your second post, it seems that this is what you applied for. The ability to apply for this grant ended in 2004.
Those that had applied for the grant (a.k.a. naturalization) under 5(2)(b) of the 1977 Act had their citizenship converted to be by descent with the 2009 Act, so since then, you would be considered a citizen by descent under
3(1)(h) of the current Citizenship Act. Under the first generation limit, your child born in 2010 would now be considered second generation by descent to you, and he/she is not eligible.
You're essentially correct in that there was change in the law. Your oldest child was able to claim citizenship because, prior to the 2009 Act, there was no generational limit; as long as the parent was a Canadian citizen (by any way) when the child was born, the child was eligible for citizenship. The generational limit was implemented in 2009, so any child born outside of Canada from a first generation citizen parent by descent after the commencement of the 2009 Act (such as your 2010 child) would be ineligible for citizenship by descent (with the exception to Crown servant grandparents I described in my first post). The 2009 and 2015 Acts did not revoke citizenship for second and subsequent generations if they were eligible for it when they were born, so that's why your oldest child is still a citizen despite being in the second generation.