Fòram na Gàidhlig

'leanabh' = child and 'os cionn' takes the genitive, so 'above a child' would be 'o chionns leinibh', would it not?

But how would you say 'above each child'? Is it as simple as 'os cionn gach leinibh'...?

Yes, os cionn gach leinibh. Regarding grammar, gach behaves pretty much like the article, this is why you also have "leis gach dùrachd", for example.