CrocsUnisex Adult Crocband Clog

I am totally confused by the sizing!!!! i need a pair for my 11 year old daughter who is a uk size 4. please can you help!!!

My son fits the m4 w6 shall i therefore order 3uk women/4uk men?

My son wear Uk 12 for Nike sports shoes. what size should he choose for Crocs?

According to the size chart i am a w7 equivalent to a uk5. when i put these new w7s in my basket it says eu39/40! so what size is a uk5??