The ruling on burying the Disbelievers in the Graveyards of the Muslims
Question :
Is it permissible to bury a disbelieving child in the graveyards of the Muslims if a Muslim adopted him, then he died before reaching the age of puberty?
Answer:
It is not permissible to bury a disbeliever in the graveyards of the Muslims, regardless of whether he was adopted by a Muslim or not, and regardless of whether he reached the age of puberty or not. However, if something is found in him (the child) that proves (or alludes to) his Islam, then he is to be buried in the graveyards of the Muslims. It also should be known that adoption is forbidden in Islam, due to Allâh's Statement:
"Call them (adopted son) by (the names of) their fathers."
Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 3 Page 81