who to I have to contact if I want to complain about ticket tarif and service?

My husband and I were traveling with our little childrren--4 yrs and 6yrs-- on July 20, to Barcelona from Paris.
We purchased a ticket for one cabin room with two lit beds. The ticket sales person said children under 11 years old can share bed with us and don't need to pay. On the ticket said "Double Familial", and we paid 516 euro.
After the train has already left from Paris, spanish ticket controller came to us and said we have to pay for the children's supplimentary ticket and charged us 120 euro. My husband tried to explain to him what the ticket office in Paris said in french. The ticket controller said he doesn't speak french and this is spanish train and there's no personnel in the train who can speak both french and spanish. He said if we don't pay 120 euro right at the spot, he will make our children get off from the train at the next stop. -----He was threatening, rude and shouting at us. No other personnel in the train was willing to help us to explain to this ticket controller about the conditions french ticket sales person had said to us. They all said " this is spanish train"
We were forced to pay 120 euro extra for children's supplimentary ticket and ended up paying total of 636 euro.
I can pay this amount for flight ticket for two adults and two children to Barcelona and have peace of mind. Next time I know what I'm going to take to go to Barcelona!
We wanted to complain the situation to spanish ticket office when we arrived in Barcelina, but do you know what they said?
" you bought the ticket from SNCF, you should contact them and ask for refund or file complainet." I should spend my own money and time to contact SNCF for terrible service I got from Spanish train personnel? I tried to contact spanish railway service through the phone they said the same thing.
Terrible, terrible experience!!
Who can I contact, to prevent things like this from happening in the future?