My son is 13 and here is a look at what he does with his trainer (not all inclusive) twice a week so we can get throwing in after a few days rest. To me this is just to start him out and get him used to it, he enjoys it so we’ve kept it up.
Almost all the squats are done with weight (goblet hold)
Chest Press (dumbell)
med ball slams
med ball throws (soccer throw in style)
And an assortment of core body exercises
When he does anything that fatigues his forearms, we avoid any throwing for a couple days.
My thing… I won’t let him touch weighted balls for throwing… we do use them to teach arm action both throwing arm and glove hand, but only slow motion to mimic the motion, not to actually throw… when I was young I hurt my arm throwing a water logged baseball… again just my bias.