There is a big difference between an airling being obliged to sit an adult next to a 2yo or to sit an adult next to a 7yo though.
I don't think that you should atutomatically expect that you have the right to six next to a 7yo on a flight where all passengers have been given the option of paying but you have chosen not to. A 2yo, yes, but any older than 5 and I think it's up to you to pay in the same way as if you wanted to sit next to your partner.
Nobody seems to be able to find any clear guidelines as to what age the airlines are advised to make sure children are still sat with parents, and everything that has been linked to is still only guidelines.
The age of the child and whether you should have to pay or not makes all the difference. As much as I would have sympathy for a parent and a 7yo who was upset because they hadn't been seated together, I would be unlikely to give up a seat to acommodate a child of that age. If you know your 7yo is likely to not be ok if s/he doesn't sit near you, then you pay. 7 years old is too old to assume that you should be given seats together because of safety. Most 7 year olds would be able to put a mask on or handle their own meal, so why should they get special treatment that others have to pay for?
When it comes to children of that age, it becomes a preference and not a safety issue, so someone elses need to sit next to a 7yo is no greater than my need to sit next to my husband. Some adults are terrified of flying and some children love it, so I don't think it's fair to say that a child of that age has more right to sit next to their travelling companion than anyone else does.
People are talking about safety issues in case of needing to evacuate the plane, which I think is ridiculous frankly because I can assure you that whether my child was 2 or 12, I would be more concerned about looking for him if we were seated apart than I would be about making sure the passengers around me had as easy an exit as possible.
People are saying that the airlines are being greedy, which I do understand, but at the same time, many passengers probably like having the option to pay to choose where they sit. I know I like it when I'm allowed to go online and choose to be at the middle front of the plane, with a middle and a window seat if it's just me and dh. I would actually be quite annoyed if the airlines took that option away from me just because some parents didn't want to pay to be sat next to their older children. It's just part of the cost of the holiday.