I see a lot of people feeding fruits regularly to their dragons. I want you to remember that with babies and younger dragons, most (around 70%) of their diet should be in the form of live foods and the remaining part should be your greens/veggies, etc. I see many people stress and say things like "how do I get my baby to eat fruit?" Honestly, I've never fed my dragons fruit, not even once. Its really just acid, sugar, and water. Yes, dragons thrive on variety. The amount of actual fruit they eat is so small that in reality, would be technically insignificant anyways. The main issue is that if you feed fruit, you don't overdo it. It shouldn't be a point of stress in a dragon's diet, for any age drgaon.
I'm not saying fruit isn't healthy. As with anything, nothing should be relied on, heavily, especially in an animal that's all of anywhere from 10 grams to 600 grams. In their very essence, fruits provide hydration and they contain natural sugar, which raises the blood sugar level. Not necessarily a bad thing, just comes down to personal preference. I prefer foods with a better mix nutritionally, for our dragons.
Blackberries, strawberries, watermelon (seeds removed) would all be decent choices. You can refer to the food chart in our group on facebook for a full breakdown of many fruits and their basic nutritional profiles.