I think the ticket is a bit of a red herring, although she was only there 30 minutes before Star was attacked, and F did say from early on that SB told Star off and F told her not to because she wouldn’t understand.
I think Star being a “sign” from her mum, and FS making noises about stopping contact was the real motive. The ticket may have been a trigger for her rage, Star maybe taking a toy or crisps from one of the other children maybe, I don’t think we’ll ever know.
I’m personally not surprised that F can’t remember everything clearly. In the interview with Dr Taylor on the day of Star’s death, Dr Taylor said F was quiet and couldn’t remember events clearly whereas SB was louder and could recall things. That said, I feel a bit more doubtful after today and I imagine the next couple of days will make very uncomfortable reading.