Nobody ever raises this concern when they increase the military budget a couple tens of billions of dollars.

It's not some big secret how you'd pay for it. You'd use taxes.

Universal childcare costs a lot more than 50 billion a year.

According to the Washington Post around $90b. https://www.washingtonpost.com/news/wonk/wp/2016/04/06/the-9...

(I feel like it goes without saying, but the idea that $90b for a program that would seriously improve many people's lives is unthinkable but another $50b for what is already, a few times over, the world's largest military budget, for a country facing no serious threats to its own security, is a trifle not even worth serious discussion is a false one, if you ask me).