One of the comments said that its Barron in the middle. The two younger boys are Ivanka I believe.
I wondered if that was Barron as the young man looks too big to be one of the grandchildren, unless they have grown as much as Barron has.
I was thinking the one in the middle looked bigger than grandkid.
They are Don Jr kids, he had posted more pictures on his Instagram page.
https://www.instagram.com/p/B8sCH83FkhC/
https://www.instagram.com/p/B8tYxfUFcFc/
https://www.instagram.com/p/B8rQwGJFKp9/
https://www.instagram.com/p/B8qxNz2FSii/