Is it permissible to sell an idle waqf (endowment) and replace it with another that is more beneficial, and can the beneficiaries of the waqf be changed to include all future children?

It is permissible to sell an endowment (waqf) and purchase something more suitable to achieve its benefit, as held by Shaykh al-Islam Ibn Taymiyyah. However, what has been specifically endowed for two daughters does not include children born to the questioner thereafter. This is because an endowment is a binding contract according to the majority of jurists. Once it is established with its conditions, it becomes binding on its owner, and the endowed property ceases to be owned by the endower, losing their right to dispose of it.