I have no opinion on the usage of Baby boom variants in German, but wanted to add a footnote on the usage of search engines to check frequencies.
The counts I get when restricting results to German only ("Baby boom" is used in French and other languages) are:
"Baby boom" 3,700
[but: Baby,boom and Baby+boom: both 19,600]
which are different than what you saw, Uho.
The reason that several counts are the same is that Google does not recognize the hyphen per se, and so searches for "Baby-boom" return citations both for "Baby boom" (no hyphen), and for "Baby-boom" (first one occurs at result #14).
Finally, periods and most other punctuation are also ignored, so in many such cases you must be on the lookout for constructions where the first search term is the last in a sentence or clause, and the second term is the first in the next one. This is not much of an issue here, as one would be hard-pressed to come up with a reasonable context of two sentences resembling "...baby. Boom..." but it can certainly affect other searches of this nature.
Moral of the story: choose your search terms carefully, restrict to the desired language, and be very careful when drawing conclusions from frequency counts.