I have travelled many times Trans Atlantic/Pacific with my son since he was an infant in J. I always tried to book side by side pods, the website allows for this, it has been an issue twice out of ~ 50 flights. The first time my son was very young ~4, it was the first time it was an issue and they made it very clear that I had to sit behind or I could offload. I choose to sit behind, my son was fine but I was very anxious. The second time it came up my son was ~11 and seated across from me on a flight from LHR to YVR, the entire plan was full and in oversold status, AC had paid passengers to get off the plane. The FA told me that it was policy that he sit in front rather than across, but that she thought he would be okay and it was not an issue, likely my status as SE, my son's age and status as 50K, and both grandparents travelling in J on the same flight may have been factors. In my experience the rule is rarely enforced. I can definitely tell you it comes as a shock to a parent to be told they have to sit in front of you, and it produces parental anxiety not to be able to see them when they are very young. IMHO parent's should be advised at the time of booking and if it is a policy it should be uniformly enforced. I can state from personal experience it is generally not enforced, but when they do enforce it they accept no arguments to the policy.