BA will show you what seats they have allocated to you at T-72 but you can't officialy change them until T-24. I have had many issues getting my daughter sat near a parent. In one case she was sat in another section on a Hi-J, we were in either 12/13/14 and she was allocated in the range 17-20. In some cases someone in the Exec Club has taken pity on us and have escalated the issue so we could sit together. Other times we have sat at T-24 sorting the problem out.