Years ago, due to weather issues, my 9 yo daughter and I were not given adjacent seats, I was couple of rows behind her. The person at the check-in desk was trying to help but the plane was full.
Finally she smiled and said "here's what you do. Board toward the end so people will already be in seats next to her. When you sit her down say "ok, honey, I'm just a couple of rows behind you. Be sure to let me know if you think you are going to get sick again "."
Sure enough, both folks offered to swap seats with me.
I now travel with grandchildren and plan to use the same line if I need to.