I’ve been away from immigration law for six months now, but I can tell you this: There is no law that requires undocumented children and parents to be separated. If there were such an absurd law, you would have known about it long ago. Family unity is a fundamental principle of our immigration laws. You find it written into all aspects of our laws.
There are various laws and regulations that require certain removable aliens to be detained. Some criminals and those who pose a terrorist or security threat must be detained. Others may be detained or have a reasonable bond set if they are a danger to the community and/or are a flight risk. A lack of strong ties to the U.S. or the lack of any legally valid basis for remaining here generally indicate that the individual may be a flight risk.
If a parent is detained for one of the above reasons, then obviously the child cannot be kept with the parent in an adult detention facility and must be placed elsewhere. Undocumented children are not kept in immigration jails. Under the Flores v. Reno class action settlement, such children must be placed in the “least restrictive” setting appropriate to their age and needs. This might be a licensed group home or foster home, if no other relative is available. They go to school, receive medical care, counseling, etc.
Homeland Security has built family detention centers with family living units, but there are tens of thousands more families than can be accommodated. And other class action lawsuits have caused some of these centers to be shut down.
Family separations are thus an unfortunate, albeit temporary, situation that comes with immigration enforcement. Such separation should never be used as a deliberate policy.