Different videos capturing the awkward moment have been spliced together into one clip -- and it makes for a tough scene for Ivanka Trump and Jared Kushner ... who were on hand Saturday to watch UFC 299 in Miami, something DT himself also crashed.

As you can see, when Trump was making his entrance ... he was hand-shaking and hobnobbing with a lot of notable people -- including Dave Portnoy -- but when he finally landed at where Ivanka and Jared were standing, things got a little icy.

Check it out ... Donald kisses Ivanka and hugs Jared, but when they try to present their son, Joseph, to his grandfather -- DT appears to turn away without acknowledging him.

The ex-Prez continues on his way without even so much as saying hello -- and Joseph is seen standing still in the immediate aftermath ... looking pretty sad. His mom goes down to seemingly console him before the clip cuts out entirely -- but on its face, it looks harsh.

Unclear if Trump was just in a hurry or what the deal was -- but in any case, he didn't give his 10-year-old grandkid the time of day ... and it seemed to hurt the boy.

We should note ... Don has been fond of and friendly with his grandchildren, in general, in the past -- including with Joseph specifically. He's been photographed holding his hand while walking across the White House lawn on at least a couple of occasions.

Ya gotta figure he loves the kid, but might've been distracted considering everything that was going on in the moment. He was arguably the most important person in the room that night.

BTW, Donald has a massive brood thanks to his five children -- Donald Jr., Ivanka, Eric, Tiffany and Barron. He has since become a grandfather 10 times over.