Cordial has a distant feeling to it. If you say two people were cordial to each other, it means merely that they did not argue, not that they were close.
Also, "relationship" (or "friendship" if you use that instead) requires an article. And since you are each your own person, I would use "lives" and not "life."
Here is how I would write it:
Since our childhood, we have had a close friendhip, and I hope it lasts throughout our lives.
Student or Learner