Can I add a baby as a co-author of a scientific paper, to protest against co-authors who haven't made any contribution?
There was a similar case in 1975, when an American mathematician and physicist, Professor Jack H. Hetherington of Michigan State University, added his cat as a co-author. Apparently, a collegaue who had reviewed his manuscript, had pointed out that he had used "we" and "us" throughout the manuscript, but this was incorrect as he was the sole author. so, instead of typing out the entire document once again (those were the days before Ctrl H), he decided to speed up the process and added the name of Chester, his pet cat as a co-author. However, to disguise the fact, he put "FDC Willard" as the name, with FD standing for "felix domesticus" and the C for "Chester." And Willard was the name of Chester's father.

However, these days, with so many regulations in place, I'm not sure if it would be ethical to include your baby's name. If however, you choose to do so, I would suggest you inform the editor and mention this in a disclaimer somewhere within the title page, so that readers are not misled about your intent.

Another well known case is the paper "In a fully H-2 incompatible chimera, T cells of donor origin can respond to minor histocompatibility antigens in association with either donor or host H-2 type." by Polly Matzinger and Galadriel Mirkwood in The Journal of Experimental Medicine, 1978.
As Ted Anton described it in his book Bold Science, "Refusing to write in the usual scientific passive voice ('steps were taken') and too insecure to write in the first person ('I took the steps'), she instead invented a coauthor": her Afghan Hound, Galadriel Mirkwood. Once discovered, papers on which she was a major author were then barred from the journal until the editor died and was replaced by another.