Here's some of the problems.
1. Under no circumstances can any child be sent, with their parents, to an adult jail if their parents are charged and booked into jail for a crime.
2. Aliens can be held, pending resolution of their cases, in a "detention center" (which is not a "jail") pending resolution of their case. Whether it's for more than 20 days, or not. Generally this has been done with immigrants who are seeking asylum and who have committed no crime.
3. However, under the Flores case, CHILDREN cannot be kept in detention centers for more than 20 days. After that time, they must be placed with a responsible adult or in a licensed child care facility. Since detention centers are not that, the law mandates that they be shipped out after 20 days (you can thank the ACLU for that).
Among other things, Trump's order seeks an amendment to the Flores settlement to allow children to be kept in detention centers with their parents, even if they have been criminally charged, and even if it is for more than 20 days:
Quote:(e) The Attorney General shall promptly file a request with the U.S. District Court for the Central District of California to modify the Settlement Agreement in Flores v. Sessions, CV 85-4544 (“Flores settlement”), in a manner that would permit the Secretary, under present resource constraints, to detain alien families together throughout the pendency of criminal proceedings for improper entry or any removal or other immigration proceedings.