You're the economist. What do you think would happen? I am honestly not sure so I'm asking you what you think based on the information given.
Well the causes of increased fertility rates aren't entirely clear. There seems to be evidence that increased incomes and stronger social programs for child rearing have helped to increase fertility rates in industrialized first world countries.
I would imagine that in your timeline the United States government would have had to enact policies to keep population growth strong in the face of such strict immigration policies. Or perhaps there's a smaller Baby boom in the 2020's as a result of millenials (sometimes referred to as echo-boomers since many are the children of baby boomers) start having their own children.
Generational patterns usually go as follows:
Silent-small Generation
Boomers-large Generation (children of the greatest generation)
Gen X-smaller Generation (children of silent generation)
Millenials-larger Generation (children of boomers)
Gen Z-smaller Generation (children of Xers)
So the next generation (I think they're referred to as Generation Alpha?) born from roughly 2018 and onwards would also likely be larger since they're the children of millenials and not Xers.
What would be interesting is, how Generation Alpha votes. Baby boomers born in the 50s became very reliable Republicans.