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 is His slave and Messenger.
Among the established rules of the Sharee'ah is that helping in the disobedience of Allaah is forbidden, as Allaah says (what means): {…but do not cooperate in sin and aggression. And fear Allaah; indeed, Allaah is severe in penalty.} [Quran 5:2]
Therefore, if a person knows or predominantly thinks that the tenant, by hiring the auditorium, intends to do what is forbidden, such as having forbidden singing and religious rituals for non-Muslims, then it is not permissible to rent it to him.
However, if the landlord does not know that the tenant will use the auditorium in what is forbidden, then it is not forbidden to rent it to him, even if the tenant does what is forbidden afterwards, because the contract was concluded regarding something permissible in principle. please, refer to fatwa 308935.
Hence, you should advise your parents and clarify the religious ruling to them.
As regards helping your parents in this activity, then it is forbidden, as we have stated above regarding the details of the lease, but mere maintenance of the auditorium and the like does not seem to be forbidden as long as it does not directly lead to what is forbidden.
For more benefit, please refer to fataawa 102612 and 22806.
Allaah knows best.