why wouldn't it be cuno? there is a canon where he becomes a detective after all
....... Because Cuno is literally piggybacking on Harry. The kid by the cake can't be Cuno, because Cuno is ALREADY in the picture. He's the red head IN THE CENTER OF THE ART wearing his short sleeved blue jacket.
Everyone in this picture is easily identifable. Mack and Chester are in the back putting up decorations, Chester is sitting on Mack's shoulders, Harry is the very center singing with his giant mutton chops, Kim is on the right closest to the camera laying out the cake, Cuno is on Harry's back, Jean with his big beard is wearing a pink bunny costume, Judit is the sole female adult, with red hear and wearing a police uniform, Trant is in the far back with his short hair, and then there's a random kid looking at the cake. The one who hasn't been identified.
They can't be Annette because Annette has blunt bangs and longer hair, and it can't be Cunoesse because Cunoesse has red hair. That kid has brown. Unless the artist got the hair color wrong and it actually is Cunoesse because they both have messy choppy hair.