It is the boy beneath who is the frog, his bent back, chin on chest, somewhat resembling a ranine attitude.
The sport of leapfrog is not recent.
Shakespeare speaks of it when, writing Henry V, in 1599, he has the king say to Katherine of France that he would marry, “If I could winne a Lady at Leape-frogge, or by vawlting into my Saddle, with my Armour on my backe.”