oswin, this explains it straightforwardly:
www.theguardian.com/us-news/2018/jun/18/why-are-families-being-separated-at-the-us-border-explainer
Or you could try what I posted way above (pls excuse the repetition).
The outcry is about a new Trump administration policy deliberately separating children from their family members or carers, then declaring them "unaccompanied minors" and keeping them in mass centres rather than individual foster homes, when the children could have stayed with their families, in family-suitable immigration detention if detention is absolutely necessary.
Yesterday Trump halted his own new policy of separating children, but there is no provision made for reuniting the over 2000 children already taken.
The story "the Dems are responsible for this" is a lie being pushed by Trump's admin and some of their more sycophantic media channels.
Your interlocutors might want to reconsider what news channels they rely on, as theirs seem to be keeping important information from them.