It fits all of your criteria.
1: Children being forced to live in a country that is not like the one they were being raised in, taken away from the only country they've ever known.
2: Children being forced to be taken away from the only school system that they've ever known.
3: Children being forced to leave behind friends.
And with the additional criteria that you just moved the goal posts to here in this post:
4: Forcibly being sent to a country that they have never known.
5: Not going to know anyone.
6: May not even speak the language.
My example fits all of your criteria. So....is it unconscionable or not?
And fyi, most immigrant families children, be they legal or illegal, speak two languages. Their parents native language and English. So if they get sent back to their home country they will more than likely be able to speak the language just fine. Even here in Idaho our schools have programs to specifically help children that don't speak English, or very little English because they were raised to speak their parents native tongue.
You really can't compare voluntary ex-pats who do an at least very often temporary stint abroad with forced deportation of 800,000 individuals, with 800,000 unique family stories.
My wife spent most of her childhood abroad because of her dad's job took them all over the world. You're ignoring several things that made that life a good one for her:
1) All the U.S. to foreign country relocations are completely VOLUNTARY. So those who accept the post are those with the temperament and support to succeed at it. Even if your employer gives you no choice, the option to quit and find another job is available.
2) Dad had a job, which means money wasn't ever an issue for them. If he lost that job, they return to the U.S. and the safety net available here, both family and our government.
3) Stable family support. Mom never worked - it wouldn't have been possible because dad was gone for weeks at a time.
4) A welcoming ex-pat community, generally including other Americans, generally special schools or special classes within foreign schools.
5) Extensive support from the sponsoring group (employer, church, non-profit org, etc.).
6) The support of the United States, such as the State Dept, should it be needed. My wife's family did need it, and it was there for them.
7) The option to bail at any time, board a plane back to the U.S., such as in case of serious illness, family emergency, just don't like it and can't cope with it.
I could keep going but the point is obvious - the voluntary decision to work abroad is NOTHING like being forcibly deported.
And as a general comment, it's pretty cowardly in my view to pretend that those affected will not face any significant problems. First of all, with 800,000 possible deportations, it's certain that some large number will be personally devastated by the act. And the bottom line is you cannot care at all what happens to them, anyway, because those rounding them up and sending them back to a country many have never really known do not care, there is no provision in the process to give them any support in the new country, etc. So just admit that their personal situation doesn't matter, if the home they left was a gang infested war zone, that's OK, send them back anyway, with no money and no family and no support. It does not matter because what matters, and the only thing that matters, is they get deported.