The NSYNC singer reposted a clip Saturday to his IG Story showing a TikTok video originally published by a man who says he's the uncle of 10-year-old Tess Marie Mata -- one of the 19 children slaughtered Tuesday by a crazed gunman at Robb Elementary School.

In the video, you see Tess dancing to Don Toliver's 'No Idea' -- a track of his which has been used a lot in different challenges, and which has become a fan fave lately among TikTokers.

Tess, clearly, was no exception ... as she had her own version to the dance -- something her uncle here says she was hopeful would translate to a little internet fame one day. He makes the plea to his followers ... can we honor her wish in death and make her go viral?

Lance was clearly on board, posting this on his own page ... and also commenting, "breaks my heart," to which the uncle thanked him -- saying Tess was a big fan of the boy band.

This child is just one of many whose life was cut short ... but it's videos like these that show, they had personalities and dreams -- all adding to the devastation, especially in light of reports that cops were slow to the draw and could've stepped in to help sooner.

People are rightly outraged over the fact current gun laws allow for weapons of war to be in the hands of ordinary citizens, but the videos of parents being held back and/or arrested in the moment while officers just stood around should, perhaps, cause as much indignation.