As discussed under Alma 14:10, also can come either before or after the past-tense verb form was. Here in Alma 20:13, we have an example involving the present-tense verb form are. Since either order is possible, we accept the earliest word order, “also are come” (the reading in 𝓟) rather than the alternative order “are also come” (introduced into the 1830 edition). For a more general discussion, see under 2 Nephi 21:13.
Summary: Restore in Alma 20:13 the original word order, with also coming before the helping verb are (“and now his children also are come amongst us”).