Latin is a language more heavy on writing, translating, and grammar. In fact it’s a great way to teach grammar! Fourth grade is when CM recommended more formal grammar, so I think that’s why she started Latin at that point. French was a language more easily taught through converstation and songs, which I imagine was the focus during those early years, before moving onto French grammar and written translations in later grades. Programs like CAP and MP are classical and tend to introduce grammar at a younger age, so it makes sense that they would introduce Latin earlier.
I’d go ahead and start with Spanish if that’s were your interest lies. The Spanish words they do learn will be similar to Latin, if you add it later on, and will be fun for them to connect the two languages.