You'd rather explore the bounds of my concession?Only those crossing from Mexico? Not other borders or points of entry?
Asking that people abide by the law of the land is not "cruelty". These are individuals who have abused the good faith of a nation to which they were invited, and their length of unlawful stay should not prevent remedial action being taken by the authorities. One cannot be said to be "law abiding" while flouting the law.
Most Americans, other than saints such as yourself, do not abide by every single law.
Even setting that aside, unless one is of the "sins of the father visited upon the children type," deporting people who have long-established families here (and are otherwise law abiding) is not fair. Really, by any measure. It is cruel, because it doesn't have to be that way.
Our system historically has allowed for mercy, our system can allow for it, and it should allow for it.