We had written agreement between the hostel and booking.com based on information written on their website, one adult and the addition of a child for 10% more. This was prearranged and agreed in writing.
When we arrived they said that they didn't care what was written on the booking, that the price was 10% discount for the child, not 10% for the child (Increasing the price from 10% to 90% of the adult), thus pushing up the priced to almost hotel prices. They have since changed the details on booking.com to 10% discount. But I booked based on the old information that was listed and agreed in writing.
In the end I had to pay, as I had no idea if I could get a hotel in the area or how long it would take, given it was evening already. I guess that if you just make a simple booking then it will ok, but don't make additional requests on booking.com as the hostel will not respect these when you arrive.
Booking.com has been marvellous in sorting this out after we arrived back, but frustrating for them and us. To get out of refunding the difference when they were challenged by booking.com, the hostel even invented another child, saying that I had arrived with an extra child that wasn't on the booking... really?
It is a hostel, but the breakfast was good.