Booked KLM flight from LAS-AMS-LAS through Delta, as I am already a skywards member. I have twin 5 year old twin traveling with me. I came to know that KLM provides a 25% discount to children on this route. I had booked it and it's been over 24hours.

Called Delta and asked about the discount, they just told me to go pound sand. They said they will not give me the discount and that I already accepted the fare and paid.

I called KLM - they informed me that Delta booked my child's ticket without the "CHD" code, even though the birthday specified was for a 5 year old. KLM also says delta got the discount of 25% on their cost, but it's up to them to pass it on to me. They want me to sort it out with delta.

Additionally - some travel agents are offering this discount and some not. Delta website does not and some 3rd party websites do not.

With this discount, I will save over $850.

What are my options? How do I navigate to get this discount with already booked tickets?
Should I raise a complaint through their website?
Should I write a letter to Kristin Colvile - Vice President – Reservation Sales & Customer Care?
Should I complain to the FTC as they are pocketing my discount and not passing it over to me?