Yes, leave them there tell the next day where you should replace them with fresh greens. When dragons are so young like that they won't each much greens. So make sure you aren't wasting a lot of greens by making a big salad. You can adjust how much salad you make through trial and error.
stems are safe. they just need to be chopped up into manageable pieces. Good advice above on misting the food bowl a couple of times throughout the day. It really helps them stay moist. Other things that can encourage salad eating is adding the good stuff like shredded squash or shredded carrots. I personally use spaghetti squash and carrots. The baby eats more dubias than greens but he still does pick through them enough to get the good stuff.
Well I am pretty much a newbie to bearded dragons so far but I have found out that feeding my young beardies Charlie and Argon their veggies first before any live food encourages them to eat a lot more than they usually would. Also I sometimes give little pieces of strawberry since I have found that it seems to entice them. Just thought that I would mention it. Oh and I never knew you could feed beardies carrots, I think I will try them when my beardies are having one of their "only insects" days.
You do not need to dust Phoenix worms. I love them because they are just about no maintenance. One thing to keep in mind is that once your dragon starts eating them (it took mine a week or so to pay any attention to them, now he loves them) they can eat a lot of them. I put about 50 in a small glass bowl (the worms can't climb up the glass and therefore stay in the bowl all day) and leave them for him all day. By the end of the day they're usually totally gone. So 100 may not last as long as you think. If you can swing it, it can actually pay to get the larger quantities, it ends up being cheaper per worm and you save on shipping. For my 4 month old, I just ordered 1000 medium worms for about $40. They come in 2 containers of 500. I use one container and put the other one in a wine cooler at about 55 degrees. They last for weeks (even months) in the cooler, ready to use when you are. That order should last almost 3 weeks or so.
I also leave some greens in his viv (he doesn't eat many greens yet, but sometimes tastes them) and I give him about 15-20 small crickets a day in the afternoon, if I'm home to do so. I'm not sure if he needs the crickets, as he eats a lot of worms, but he seems to love catching them so I figure it's a good thing.