God promised to make a great nation of Ishmael. Scripture even records that he had 12 sons, all of whom are indicated as "princes". (Genesis 25:15). Ishmael had Abraham as his father, the same as Isaac did, but the promise was through Isaac, not through Ishmael.
You need to realize that it is those of faith who are the true "children of Abraham". Even Jesus indicated that when he told the Jews that "I tell you that God is able from these stones to raise up children of Abraham".
"Abraham believed God and it was reckoned to him as righteousness". Do you believe God that He sent His only Begotten Son to be the sacrifice for our sins? If the answer is "yes" then you've become one of the children of promise of Abraham.