Is it permissible to take a ritually impure (najis) mattress to the laundry for washing without informing the worker of its impurity, given the difficulty of washing it at home?

There is no harm in offering a ritually impure mattress for washing, as long as it is dry, the impurity will not transfer from it. The worker will wash it, and it will become pure. It is not a condition to inform the worker of its impurity, as no harm results from not doing so.