I sympathize with the the sentiments of the poster.
Unfortunately the coach/s are charged with picking a balanced TEAM, one that has a balance of bigs, forwards, guards & point guards. As unfortunate as that is, that means there are generally only two spots for each position, so if your child is not one of the top two players for a certain position, then your child is more than likely going to miss out.
However, making junior teams is not the be all. Kyle Adnam never made a state team and is now playing for Melbourne United and there are others that made nearly every state team in their time in Juniors, yet never played Big V, SEABL or beyond once out of juniors.
Dominating in juniors, whilst nice if your child can do it, should not be the goal. The journey should be geared to where they are at by the early mid twenties.
Junior sport is so fickle as some kids mature early, some late. Generally the early maturing kids have an extreme advantage, that generally diminishes as the kids age. Things only start to even out at top age 18 of the U20's.
I have two kids, one made every state team going, the other never made a state team. Yes I was disappointed, a little like the original poster, but looking back it is not such a big deal, he has played in SEABL when most of the players that made those state junior teams are no longer playing or were/are not good enough to make Big V or SEABL.
So my advice is train hard, work on your game and if you are good enough, like Kyle Adnam, your child will make it in the end.