The biggesr problem is she's tall and skinny so need a 7-8 or even 8-9 for the lenght but 5-6 for the width.
Mostly buy then on the small size cheap in the summer. She's seem to be growing again her binki elastic has got and the swimsuit is cutting her in to her top and bottom. None of the cheap supermarket swimsuits avaible at this time of year.