From the article, the family was driving from Rio Grande to Houston for her treatment, going through an immigration checkpoint, as they have done in the past but this time the entire family was detained and deported on the spot.
Generally people with family who have citizenship and noncriminals are all exceptions, they used to be called undocumented instead of illegals because even undocumented workers pay taxes in the USA. Not anymore, now anybody and everybody is at risk of deportation. They're even discussing removal of citizenship from whoever they like, as they attempted in Trump's last term.
It is worth noting that because of this policy the ICE is actually less effective at catching actual criminals.
They're called "undocumented" because it is not a criminal offense to enter the country, but a civil one. Some also use "unauthorized" which is probably a little more accurate.
They deported the parents. Would you leave a sick children under the custody of a state who thinks you're sub-human due to your skin color? They had no choice but to bring the kid back with them to Mexico.
I've been on about that for about 3x as long, and roundly met with blank stares and outright denial. Once again, not at all happy about being proven correct.