Having given women the breasts of little girls, the next logical step was to give them bodies to match. But Jack went one step farther- the new mutant Girly Virus would turn both men and women into pretty little girls- the effect was random, but someone affected could end up as young as 6 or merely a tween. Puberty would be reversed regardless.
Men transformed would become wholly female, even on a genetic level. Children below 6 wouldn't get older, but boys would turn into girls. With the whole of humanity becoming unable to procreate (as all sperm in the world would be rendered inert) and pregnancies being dangerous, even if they were possible, the disease would eventually lead to humanity's extinction. Pregnant women infected by the virus wouldn't lose their children, and they would stop regressing before it became dangerous for them to give birth, only for the virus to proceed once the had done so.
Now, Jack had no intention of wiping out the human race, so he wished for several scientists around the world to eventually get the idea for new cloning technology that could take the DNA from two people and create a child from that (which, technically, the potential for such already existed, so Jack's intervention would just make the process more efficient), as well as the idea for an 'artificial womb' that would safely allow the child to grow to infancy safely without any health problems. Again, just a fancy and very efficient incubator.
Then he sat back and watched the chaos. World leaders and celebrities turning into cute little girls the world over, and issues all around as people became smaller, in some cases, too small to effectively use their own technology. But the world quickly adapted to this new normal.
As for Jack himself, he was now Jackie, and a pretty girl of about 9. All in all, the experience hadn't been unpleasant, although his relationship with his girlfriend had changed somewhat...