Is it permissible for those in charge of a Quran memorization center, which relies for its funding on an attached project for hosting children, to take salaries from the center's income?

If the center is not an endowment (waqf) or based on donations, and its administrators work without expectation of payment, then there is no harm in them taking for themselves any surplus from the guesthouse's income. However, it is better for them to remain working without expectation of payment and not take anything, in order to preserve their reward and magnify their recompense.