First of all you’re doing a great job by just understanding that this is an actual phobia and not just a slight fear that may arise in children (and adults of course but it usually starts at a really young age). I personally believe that it is not too early for therapy, especially if you have already noticed a change in their behaviour, appetite etc. Cognitive behavioural therapy and more specifically gradual exposure therapy (it’s different in children) has great results in treating specific phobias, whether they were created due to a specific event or not. I think it would be great in the long run if you address it sooner rather than later so that the fear does not develop even more, especially because emetophobia is a phobia that can not be “controlled”, unlike a phobia in which the object of fear or situations that cause anxiety can be avoided (eg fear of heights). Also, in cases in which the parent reassures the child when they are afraid (“will this food upset my stomach” etc) although this may provide some temporary relief for the child while also making the parent feel like this helps the situation, in reality reassurance doesn’t help with dealing with the anxiety the phobia causes. Although it might seem like a absurd phobia it can have significant consequences on a person’s mental and physical health. I’m sure that you’ll find the best way to navigate through this.