Are all the descendants of Abraham children of the promise?

No. Scripture distinguishes Abraham's descendants from children of the promise. Ishmael was blessed and made fruitful, a great nation with twelve princes, but God established His covenant with Isaac, whom Sarah would bear. Only those descending through Isaac, Jacob, and the twelve tribes are counted children of the promise.

Les teaches that Ishmael was Abraham's son, and God heard Abraham concerning him. God blessed Ishmael, promised to make him fruitful, multiply him exceedingly, give him twelve princes, and make him a great nation. Les notes God did this: the Arabs outnumber the Jews 50 to 1 and have all the oil. Yet the covenant was not established through Ishmael. God said His covenant would be established with Isaac, whom Sarah would bear at the set time in the next year. So, going back to Romans, not all the children of Abraham are children of the promise. Only those who came through Isaac, Jacob, and the twelve tribes are counted as children of the promise.

Romans 9:6

Not as though the word of God hath taken none effect. For they are not all Israel, which are of Israel:

See the rest

In Romans 9, who counts as the true children of the promise?

According to Romans 9, the true children of the promise are not all Abraham's descendants. They are specifically those who came through Isaac, Jacob, and the twelve tribes. Ishmael, though blessed, was not included in the covenant line; God established His covenant with Isaac.