I read your title as "I failed as a child" 😂 But, luckily, that wasn't the real title...
Maybe she waved to the little boy behind your seats?
Jokes aside, children are indeed so pure and innocent and sometimes we don't pay attention to these small things that could be important to them... Just had a similar situation at home, but the kid is a bit older than 10... Adolescent, he doesn't need anyone... But, when we forget about something important to him, it's the end of the world... :) Not making excuses for forgetting things, as I'm guilty as charged... Luckily, we fixed the issue...
Children do understand that grownups have their issues and worries, so I would say that the little girl has forgiven you for sure... We can't protect them from everything as much as we would like to...
I had to quickly go and check my title! Phew!
I just feel so bad that I ignored her. She deserved better, and I can't get over it. Dunno why I feel this way, I don't know her, is unlikely ever to see her again and it was just a minute of my life. This isn't how we should treat a child, lesson learnt and to remember for me.
That's the most important! To learn from it and try to do better next time!