I can tell you what I have witnessed, as a starting point. There is always a person who will be able to board with the wheelchair passenger. That person is kind of like the caregiver to help them board and get situated. They are part of the "pre-Board" process, and need a blue packet that holds thier ticket. As for a long line of other passengers in the family who want to get on the plane as well as part of the pre-board, that won't happen. For kids, it is the 1-2 parents who are flying with the toddlers who are able to board during the family boarding between the A and B group. If there is another child with that family, of course they will go along. If you have other adults, mothers in law, aunts and uncles, they will board using their normal boarding group assignments. They don't "need" to be directly with the family. The general rule of thumb seems to be the disabled stay with their primary caregiver, and the toddlers stay with their parents. All the other relatives or friends board at their designated time and sit wherever they choose. There are often plenty of open seats available near toddlers!
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As long as you are not saving an Exit row seat, you are usually OK saving a single seat next to you. It is not cool if you save an Exit row seat, as people pay extra for that opportunity. There is an unwritten code of seat saving, and that is a no-no. Besides, if he is 12, he can't sit in that exit row anyways. If you take a row in the back half of the plane and save the middle seat for your kid, people will move on and it is not a problem.
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