All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
If it is proven that this stadium playground is a usurped piece of land, then it is inadmissible to perform prayer on it. An-Nawawi said in Al-Majmoo‘: "There is scholarly consensus that praying on usurped land is prohibited. However, it (the prayer) is valid according to our School of Fiqh (the Shaafi'i School), the majority of Muslim jurists, and the scholars of Usool Al-Fiqh (Principles of Islamic Jurisprudence). Ahmad ibn Hanbal and Al-Jubbaa’i, and others from the Mu'tazilah (Mu‘tazilites) held that prayer on usurped land is invalid.”
Allaah Knows best.