Good point.
For all genetic Jews are part of genetic Israel (Romans 9:3-5), but being part of the true, spiritual Israel, the true, spiritual seed of Abraham, the promised seed, is not based on genetics (Romans 9:6-24), but on God's election (Romans 9:11), which includes both some Jews and some Gentiles (Romans 9:24).
All Jewish and Gentile Christians are part of the true Israel (Ephesians 2:12,19, Romans 11:17,24, Revelation 21:9,12; 1 Peter 2:9-10, John 10:16), the seed of Abraham (Galatians 3:28-29, Romans 4:16-17), the promised seed, just as Isaac was (Galatians 4:28). And so all Jewish and Gentile Christians are heirs of all of the promises made by God to Israel (Ephesians 3:6, Ephesians 2:12,19, Galatians 3:29, Romans 15:27).
In Romans 9:8, by "the children of the flesh", the apostle Paul means genetic Jews, who are the genetic children of Abraham (Romans 11:1, Acts 13:26, John 8:37). And by "the children of God"/"the children of the promise", Paul means the elect, both some Jews and some Gentiles (Romans 9:24, Galatians 4:28). Romans 9:6-8 means that not all Jews are elect (John 8:37-47, John 10:26) and that some Gentiles are elect (Romans 9:24, John 10:16, John 11:52). Only a remnant of genetic Israel is elect (Romans 9:27), just as only a remnant of humanity in general is elect (chosen) (Matthew 22:14, Matthew 7:14).