The 1852 LDS edition introduced the definite article the before heirs, which is quite unnecessary and probably represents a typo. There is one other example in the Book of Mormon text of heirs, and it reads without any definite article:
Notice that both Mosiah 15:11 and 4 Nephi 1:17 refer to the people of God as the offspring of Christ and heirs of the kingdom of God.
The occurrence of heirs without any the is common in the New Testament epistles:
There are also two instances in the New Testament of the heirs, both in Hebrews (6:17 and 11:9). In any event, the use of heirs without the definite article the is acceptable—in fact, preferred—in the Book of Mormon text.
Summary: Remove the definite article the from before heirs in Mosiah 15:11 (“they are heirs of the kingdom of God”); the intrusive the does not conform to normal scriptural usage.