According to Article 2, Section 1(a) an foreign child had to be younger than 5 in order to gain citizenship through the adoption process. Even if such an adoption had happened, Obama was 5 or 6 when Soetoro took the family back to Indonesia. Obama would have had to wait until he was 18 and be naturalized in order to get Indonesian citizenship.
A newspaper story showed that the Soetoro’s married in 1965. Obama was four and could have been adopted and become a citizen without the need for naturalization. The story about him being 5 or 6 came out later.