It is up to the employee to request compassionate leave and the employer should allow the worker time off to deal with family emergencies. The question is how much? The law isn't very helpful and simply states that the employer should be "reasonable". If the nanny's family live in the same city or county one or two days may be considered reasonable, but if the employee is from Australia the matter is clearly different and they may require several weeks off. In this instance the worker may be required to use some of their holiday entitlement.
From a contractual point of view the employer does not have to pay their employee for compassionate leave, any payment is made at their absolute discretion.