In a teshuva written in 1973, Rav Moshe Feinstein was asked by Rav Dov Baer Baumgarten from Buenos Aires about issues involved with a couple that was adopting an Argentinian child. Rav Moshe ruled that the child needed to be converted, since most people in Argentina are non-Jewish and thus within every city both the inhabitants and those who pass through can be assumed to be non-Jewish. Rav Moshe ruled out the possibility that the infant would be considered to be an אסופי (literally someone collected from off the street whose Jewish status is thus unknown), since the presence of adoption agencies mitigates this fear.
Rav Moshe pointed out that the child can be converted as an infant and doing so is considered a זכות for him (and therefore it can be done without his knowledge or consent). However, he reminds Rav Baumgarten that upon turning bar mitzvah the child has to be informed that he was converted [note: in such a case, the child has the option of leaving Judaism on the spot. However, once he is informed at the time of his bar mitzvah that he converted and decides to remain Jewish, he can no longer turn back.]