Unless the plan is to try and outbreed conservatives, leftists will need a different strategy to avoid being outnumbered.
Parenting is very resource intensive, both in time and material resources, and is thus an inefficient means of transmitting ideologies. You could likely use that time and resources for education and outreach, influencing a much wider audience.
No strategy is perfect, but even places like Russia and China have floundered in bringing up fertility rates.
The irony is that the historically effective way to manage below-replacement rates is via immigration. But that's sort of the crux of the post.