We become the children of Abraham by being baptized into the name of Jesus Christ
Galatians 3:27-29 For as many of you as have been baptized into Christ have put on Christ. 28There is neither Jew nor Greek, there is neither bond nor free, there is neither male nor female: for ye are all one in Christ Jesus. 29And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.
Galatians 3:7 They which are of faith, the same are children of Abraham