Some kids try to move to the black (or green) plyo ball before their body is ready and cause serious injuries. Every trainer I’ve talked to over the years has said the athlete need a good foundation of strength, especially in the shoulder and arm before starting with weighted balls. Ask the trainer what he thinks. Most that I’ve talked to will try to add arm strength through methods like weight training and mechanics and only go to weighted balls as a last resort.
As far as food, their body will respond to what is put into it. If they use sugary peanut butter and uncrustables to gain weight it won’t be as effective as clean food (chicken, beef, fish, rice, etc.). Daves Killer Bagels are a great source of better carbs, chipotle bowls with extra meat are awesome, smoothies with protein, etc. Also look at the ingredients for everything. The more ingredients the worse it is. Naked Protein or Klean Protein are two of the best you can buy.
We never did the prep thing, but it is a good idea if you have time. Again, this has to be the player wanting to do it, not the parent.