The Kaf Hahayyim brings down that if one hears a proper blessing recited by a Yishma'el, even though it is recited in Arabic, one answers Amen. The reason is because his intention is to bless G-d and not Abodah Zarah (idolatry).
We mentioned that if a small child is learning to recite a blessing, we do not answer Amen. However, when a child who is of the age of instruction (Hinnukh), says a Berakha (blessing) in order to fulfill his obligation of reciting a blessing, we do answer Amen. The age of instruction depends on the individual child, and is generally around the age of six. Even though the adult answers Amen, he does not fulfill his obligation with the child's blessing, since a child is not actually obligated in the commandment.
Additionally, it says in the Shulhan 'Arukh, that when a child who has reached the age of instruction, recites the blessings on the Haftarah in the Synagogue, one also answers Amen. This is because a child is permitted to read the Maftir and Haftarah in the Synagogue on Shabbath.
(See Shulhan 'Arukh 215:3. Kaf Hahayyim ibid., oth 15, 18, 20)
If an Arab recites a Bracha, answering Amen to Yishmael, Answering Amen to a child, halacha, halochos