Note you won’t be able to do this with flying creatures.
This is a good point. Circle of Flame only triggers at the moment when a creature without flying is declared as an attacker. Casting Turn to Frog on a creature with flying to make it lose flying after it’s already been declared attacking won’t cause Circle of Flame to trigger.
You need the creature to lose flying before it’s declared as an attacker in order to trigger Circle of Flame.