I wouldn't worry. I think it's a good thing to be interacting with kids from different age groups, rather than only having friends your own age. If the girl is nice she is probably a good little role model for her too and someone to look up to. Maybe she feels safe with her while she's finding her feet at school? I think it will sort it's self out anyway. She has to spend all of the class time with her own class so it's not that much time with this other girl anyway. I would only worry if the girl started to cause and problems somehow but for now it sounds like it's all working out for both of them