As a kid, I used to play footy, basketball and cricket with varying degrees of success. I was never very good at footy in particular, but persisted with it on the insistance of my dad. I spent much of the game watching the ball fly in one direction or another over my head. I'd get frustrated when I felt players didn't pass it to me when I felt they could have. I also spent more than my fair share on the bench. Looking back though, I honestly believe it was good for me. I learnt at a young age that nothing is given, it is earned.
On the other hand, I was much better at basketball. Sure, I was never quite quick enough to be as short as I was, but I could more than hold up my role in a team. Also I found that many of the skills I learned playing footy were applicable to basketball (positioning to win rebounds for example).
I never really went anywhere as a basketball player (I still like to think that winning MVP in an u/16 a grade comp was a massive achievement, but in reality, it's not), but I playing gave me the opportunity to take up refereeing and have been lucky to be taken around Aus to referee.
To this day, I think the lessons I learned playing footy help me in some way each day. It's ok not to be the best at everything, you give something your all and one way or another you'll get what you deserve, even if it's not in the form you think.
I'd encourage your kids to stick with the hockey, encourage them to earn their position in the team, offer them support when they're down and applaud them when they do well.
If the coach is a good one, he'll be working with them at training to improve their skills and to help them win a place in the side. If he's not, then find a coach that will.