I wouldn't say anything.
I think there are lots of valid reasons for a child to not go and do prep early. Nothing to do with academics or socialisation but maturation. It does sound like this girl may not be ready for prep due to her maturation and for that reason I think it's wise to keep her from starting prep too early. Children who lack the maturity of their peers in prep can really struggle and this has ramifications for self esteem and confidence.