You could all read Huxley's Brave New World if you are into the topic of genetically engineered slaves.

To be honest, gene modified babies have to be made the same way assisted contraception is being done, which limits the spread of this practice. For it to become widespread, you'd have to make the regular way of making babies obsolete, which I'm quite sure is impossible.

It's NOT a thing you can do to already developed humans, unless you'd like to rewrite the dna of the bazillion cells that make up a human body.