The reasons why there are mostly latin-american (immigrant) families in this Spanish jail may be:

Spaniards might already have broader social circles or wider families rooted in Madrid. These may take care of the kids when both parents are in prison.

Mom and dad in prison means a messy family most of the time, unsuitable for kids all along.

This is not the case. The most prevalent crime for this kind of functioning families may be one-time drug-smuggling. This crime is frequent in some immigration routes to Spain.

The highest crime rate for migrants in Spain does not go to latin-americans, but they may have more structured families than other groups.

Latinoamerican[o, a] (Sp.) Spanish-speaking Northern- or Sourthern-American. Borrowed from the French, which in some former time may probably have meant French-speaking people like Haitians and Quebecois, too.

Latino (Sp.) 1. -> Latinoamericano (In Spain this is what you mean by Latino 99.9% of the time). 2. Person from the region of Lazio, Italy.

América (Sp.) The landmass of Northern and Southern America.