The incident went down Tuesday night at the popular MXDC Cocina Mexicana Restaurant near the White House ... where Nielsen sat in the back surrounded by her security detail and waited for dinner.

That's when a group from the Metro DC Democratic Socialists of America showered her with boos and grilled her about Trump's inhumane policy to separate children from their parents crossing the border.

The group asked her, "Aren't you a mother too? How do you sleep at night?" and "Do you hear babies crying?" The group also chanted "shame" and "end family separation" ... and 10 minutes into the ordeal, it proved too much for Nielsen to handle because she bounced.