How good are you at making gumpaste figures? It's not that hard and there are lots of tutorials on YouTube and places like that. If it were me, I'd do a little girl bending over to kiss a frog with a crown (who, in turn, is stretching up to meet her lips). Or, if you're better at piping, you could draw something like that and pipe it directly onto the cake or do a frozen buttercream transfer. Whatever you come up with, good luck!
what about making a doll cake of the princess holding the frog in her hands like this....
http://www.google.com/imgres?imgurl=http://krazzykamaali.files.wordpress.com/2009/05/tiana-the-princess-and-the-frog.jpg&imgrefurl=http://krazzykamaali.wordpress.com/2009/05/18/female-brown-anime-characters/tiana-the-princess-and-the-frog/&usg=__87sVjphD_pU3dN6ReJN-VOXDvaQ=&h=626&w=470&sz=36&hl=en&start=7&um=1&itbs=1&tbnid=VPXVPt7XRl0YQM:&tbnh=136&tbnw=102&prev=/images%3Fq%3Dthe%2Bprincess%2Band%2Bthe%2Bfrog%26um%3D1%26hl%3Den%26sa%3DN%26rlz%3D1R2ADBR_enUS367%26tbs%3Disch:1
http://popcornandasoda.files.wordpress.com/2009/08/the-princess-and-the-frog.jpg
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