I would like to make a donation to the child I adopted as part of a simple adoption. Can he benefit from the reduction of 100,000 euros?

No. The reduction of 100,000 euros is applicable in relations between ascendants and their children (art. 779 CGI).
Article 786 of the General Tax Code specifies that, except in special cases, the relationship resulting from a simple adoption is not taken into account for the collection of transfer du-ties free of charge.

Consequently, if a natural family relationship exists between the adopter and the adoptee, the applicable rate will be determined according to this relationship. Failing this, the dona-tion will be taxed as a transfer of property between two unrelated persons (art. 777 CGI, table III).