Long Toss will help a lot, a weighted ball can help but a weighted ball should never be used for long distance throwing just for warm up. proper stretching will make a huge difference there are a lot of players that only give a half hearted effort when stretching and never receive the full benefits.
You can break the motion down in chunks. Work on foot placement/shift, ball transfer then throw and follow through.
My dd did her workouts and teaches her students in gear...
At the younger ages a lot of the time the gear is not fitted properly or the players do not practice in gear so, during game time they change their motions.
Have your dd Jump Rope to get the legs strong and other polymeric core and leg training including getting a squeeze ball to get the hands stronger and bands for the arms/shoulders.
Get a ladder and cones and do agility drills to get quick feet and lower body movement.Squat Thrust, Frog Jumps and wall sits also help develop the player.
Working on snap throws to 1st and third is a must there are a ton of outs at these bases that never happen because kids get conditioned to throw only on steals to second. Doing this also keeps the runners closer for more success when they do attempt.
ACE has some pretty good videos as well.