A court has ruled the girl can remain anonymous if the case goes ahead.
The action is being supported by Anne Longfield, the children's commissioner for England. She believes TikTok has broken UK and EU data protection laws.
TikTok said it had "robust policies" in place to protect children and did not allow under-13s to join.
Ms Longfield hopes the case will lead to greater protective measures for under-16s who use TikTok in England and possibly beyond.