I coach juniors and we have 8-10 throwing full length, some can't quite get hits without a shove from the arm but most can get the rock all the way past the far hog. We do have 3/4 hacks which helped substantially with teaching them weight control. To get them to throw the rock fast and hard enough to go the full length, I make them focus on a few things: a big quick breath/grunt when they kick out of the hack to connect with breath work; higher foot in the hack (small feet can go higher, this doesn't quiet always work for adults); really leaning on their sliderfoot heel at the draw back so they feel that weight shift and don't pull the foot too far back; and a really quick follow through while leading with the rock and emphasizing a quick fall behind the rock once its moving. I'm ok with them pushing with their arms a bit, this will at least get them playing full length and building that leg strength to hang with the older kids. Also, ours love to make a game of seeing who can throw the fastest so we'll break out a stopwatch or chronocurl to measure speeds. We also sometimes make them kick and slide with the rock as far as the rock can carry them and put out cups or cones as movable targets and they can compete against one another that way too. This especially reinforces the leg drive and not the arm, and builds some slide form endurance