That would be why your beardies aren't interested in vegetables. If a 5 year old were given the choice between eating cheeseburgers every day or salads every day, what would they choose? The cheeseburgers, right? It's the same way with bearded dragons. Feeder insects are like cheeseburgers. They provide lots of calories and protein for the rapid growth that needs to happen for young beardies, and plus they taste good. But when a bearded dragon reaches adulthood, its metabolism slows down and it doesn't need as much protein in its diet anymore.
In fact, continuing to feed bugs daily through adulthood overloads a bearded dragon's body with protein, and can cause severe kidney problems for it later in life (ex: gout) and even death in some cases. Adult bearded dragons really only need to be fed insects once a week — high quality salad like collard greens, dandelion greens, mustard greens, etc. can be provided every day otherwise.
Your beardies might have a little trouble adjusting to the new diet, so what you'll have to do is give them no choice but to start eating salad. You can withhold feeder insects for up to 2 weeks without causing severe negative effects for your beardies' health. During this time, they will become hungry enough to start eating salad. I know it seems cruel, but it's the best thing to do in the long run.