Nice, don't forget its quality over quanity.
Doing this for a warm up is good, but don't focus on this too much, just let them play with abit of guidance but not too much as they need to build their creative sides and speed of thought by playing with some freedom and individual intuition, especially the younger ones, you've got the right idea with putting dedication and hard work into something to see improvements but its got to be many areas of the game and not just juggling.
Also belive me the number of kickups you can do doesn't represent how creative you can be, there can be a guy who can only do 30 kickups, but has more confidence and intelligence with his touch on the pitch than someone who can do 200 kickups, its a game situation at the end of the day but for warming up juggling is always a good idea.
The kids you train come only a few times a week right? so its important to tell them to practice and play football as much as they can, things like hitting the ball against a wall is also something you can do when your not playing with your mates, don't make it too serious like a job because at the end of the day its a game. When they get to 14/15 upwards it becomes more serious and the mental/physical development gets more importance but the talent should always be looked at first before anything.