Booking Price not guaranteed! Hidden charges on the day of check in

I made a booking for 2 adults and one child, on booking.com. it asked me my child's age at the time of the visit I said 12 then the total was R927 I don't know the exact amount which I was okay with. I always booked with booking .com and never had this problem. When I got to the holiday home only 2 adults were booked and not my daughter I had to pay for her. The lady actually said it's okay but we felt bad for the mistake booking.com made and paid in total R1200+ all I know it's R250. I do have the slips and I want booking.com to refund. I was sick so I could not call booking .com but I don't think this is fair.