Even though Belgium accepts and recognizes dual nationality, this is not the case for the Chinese authorities. Article 3 of Chinese nationality law promulgated on 10/09/1980 states:
"The People's Republic of China does not recognize dual nationality for any Chinese national".
The Belgian authorities can not issue a Schengen visa in the foreign passports of European citizens but the minor child holding a Belgian passport who has been registered in the hukou of his Chinese parent can apply for a permit to enter and leave the country with his Belgian passport at the "Public Security Bureau (PSB) - Entry & Exit Administration". With this valid "Exit Entry Permit" and a valid Belgian passport, a person can travel from China to the Schengen countries and return to China. If the "Exit Entry Permit" expires during a stay in Belgium, a new copy can be requested from the Chinese Embassy in Brussels: http://be.china-embassy.org/eng/
The issuance of this "Exit Entry Permit" is the competence of municipal or provincial authorities, it is advisable to contact the PSB of the place of issue of the child's hukou.