Moral development is severely affected by the people who are closest to you. My parents played the biggest role in my moral development and I am very happy for that. Growing up playing baseball my dad coached our team almost every year. I think parents play the largest role in moral development because they are closest to their kids and are the primary adult figures in the kids lives. Being around the kids and the fact that they are their parents plays a big role in how their morals are developed.
However, coaches play a big role in the moral development of athletes and should be aware of this. A coaches job is to coach their athletes but also to try and teach them life skills that they can use. Being a morally sound person is a important part of being a coach. Being a "good person" both in competition and life is important and can reflect on the athletes. Athletes will often repeat the actions of the coach even if they are wrong in order to win. It is important to play and coach the game the correct way in order to reflect positive morals towards your team.
Overall it is important in moral development for parents and coaches to have good morals. These are the main adult figures that a child interacts with growing up and both of these groups have a large impact on the future of that child. It is important for coaches to have good morals and carry them into the arena.