IIRC, tens of thousands of them were missing during Obama’s unaccompanied minor crisis that he pushed through.
https://www.cnn.com/2014/06/09/us/undocumented-children-immigrants/index.html
There were 65,000 unaccompanied minors in the wave of 2015. Federal officials refused to let state officials responsible for child safety onto federal compounds. Later, the Obama Administration claimed that only 52,000 crossed over. The other 13,000 were never accounted for.