Cheapest and best way is to complete the application online yourself. Start now, because it can take weeks. If you use an agent you still have to answer all the same questions and get all the same documents together. So do it yourself. They still have to complete the same application just on your behalf. So it takes even longer and costs more.
never been stopped travelling with my grandchildren. And they have different surname to me. So which article of the Hague convention says child needs letter from both parents / other parent to travel ?