No evidence given. This is a recurring pro-Kremlin disinformation narrative about migrants and Islamisation.
There is no data proving the claim that the majority of infants currently born in Brussels are from the new Islamic community. According to the Brussels Institute for Statistics and Analysis, the precise data cannot be acquired. However, data from the Leibniz Institute for the Social Sciences shows that the dominant religion in Belgium is Catholicism (52,9% in 2015), while there are only 5,2% Muslims in the country.
According to official statistics, the number of people residing in an EU Member State with citizenship of a non-member country on 1 January 2018 was 22,3 million, representing merely 4,4% of the EU-28 population. In addition, there were 17,6 million persons living in a given EU Member State as of 1 January 2018 with citizenship of another EU Member State.
In absolute terms, the largest numbers of non-nationals living in the EU Member States on 1 January 2018 were found in Germany (9,7 million persons), the United Kingdom (6,3 million), Italy (5,1 million), France (4,7 million) and Spain (4,6 million).
Similar disinformation cases can be found here and here.