The new policy was enacted in May. https://www.justice.gov/opa/speech/attorney-general-sessions... The policy refers all cases of illegal crossings to prosecution. Previous "zero-tolerance" policies have been enacted, but they did not apply to families which would be broken up when the adult(s) were prosecuted.

Edit: I might as well mention here that a 1997 court decision in Flores v. Reno (which you should Google for details) (yes, that goes back to the Clinton administration!) limits detention of children to 20 days even if you house them with their parents. This new EO doesn't even mention that decision so this will probably be immediately challenged in court.