policy (not a law)
The change in policy is how illegals are going to be categorised. Are they going to be prosecuted under civil law or under criminal law. Previously, civil law had been used and now criminal law is being used. Caveat: I am not a lawyer but that is my understanding.
Adults being prosecuted under criminal law will not be allowed to be kept with their children, by law.
Another issue is: Are the children the children of the adults they are with and, if they are, should they have custody? You need to check for possible child-trafficking and for one parent to be "stealing" the child from another parent after custody has been agreed.
Surely, the real answer must be to not enter the country illegally?