Can the rental value of an apartment endowed as housing for a poor person be counted towards the obligatory Zakat of the heirs, and anything exceeding Zakat be considered charity?

The endowment (waqf) by your brother's children is not valid because they are not adults (rashidin). Nor is it permissible for you to create the endowment on their behalf, because you are not allowed to dispose of their money except in a manner that is most beneficial to them. It is not permissible for you to donate any part of it, whether as an endowment, a gift, or charity. If you do donate, you will be held liable. Therefore, you are not permitted to endow this apartment. When your brother's children reach adulthood, they may dispose of their money as they wish. The method you mentioned is not a valid way to discharge zakat (definition).