Chimalpahin used "hijos naturales" to write about so-called illegitimate children, Susan Schroeder reminds of of related words in Nahuatl: ichtacaconetl, ichtacapiltontli, ichtacapilli, usually associated with some form of huehueyotl or huehuentzin, the means of which are not completely clear. These terms distinguished certain offspring of the kings' wives and consorts (other than incalpanpilhuan), those outside of traditional class boundaries.
Susan Schroeder, "The Noblewomen of Chalco," Estudios de Cultural Náhuatl 22 (1992), 45–86, for this example, see 78–9.
ichtacaconetl, calpan pilli, calpan conetl = the secret child, the bastard, the bastard (central Mexico, sixteenth century)
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 10 -- The People, No. 14, Part 11, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1961), 2.