Castro v United States.
Citizen daughter was detained, placed in a holding cell and sent with her alien father to Mexico.
The father wanted to take his daughter. The Border Patrol let him; against the wishes of her US citizen mother.
Both were placed on the deportation bus.
What that shows is that the Border Patrol prioritized letting the parent with (apparent) custody keep the child. The child was not deported; rather, the deported father was permitted to take the child with him. The baby, presumably, did not express an opinion.
This would not happen if both parents were US citizens.
That's because neither parent would be subject to deportation if they were both US citizens. Duh.