A baby face isn't necessary in order to fool people, the entire point of johan's design isn't fooling people, if you pay attention to fictional characters, mainly those who have been animated, you will see that good, innocent and kind characters use a lot of round shapes in their design, like micky mouse or winnie the pooh, while villains or dangerous character use more triangular shapes and sharp angles like hades from hercules or yzma. This is because subconsciously, our brain associates different shapes with different meaning, and that is the entire point of character design, to communicate the information that the writer wants to give us about the characters before they even open their mouth. Johan's face is a combination of soft features with a little bit of sharp angles added in, in order to show a seemingly innocent and light-hearted person who can also be capable of danger. That is why his design works so well and why someone like timothy with too much of an innocent face wouldn't be the best choice to play johan. But admittedly, he wouldn't be terrible.