Bit of background before I get into the AIBU.
DS (6) plays for a local U7’s football team. The team has been together for about 2 months, they train one night a week and then play games on a Sunday. Unlike most of the other boys on the team, DS literally only started showing an interest in football a few weeks before the team was formed, he’d never played before and we’re not a sporty family at all - DH/DS1/Uncles etc have no interest in football. As a consequence, he is probably their weakest player.
I’m aware of this and have been helping him practice every day, signed him up for after school football club etc as he absolutely loves to play. He tries hard and is actually great at tackling. He never gives up or gets despondent, despite always being the first one to be subbed out.
Allegedly it’s non competitive at this age. The coach is always going on about it being about building skills, having fun, learning the rules etc, it’s not about winning. I say “allegedly” because of course everyone still expects them to win, especially the coach who only ever lets my kid play a few minutes “so everyone gets a turn” but the better players never have to be subs 
On to my AIBU (finally!)
At this weeks match DS played a grand total of 4 minutes (of a possible 20)- so not even a full half - whilst several other players played the full 20 and even the others who were subbed managed to play at least double what DS did. I was annoyed because how will he ever get better if he’s barely allowed to play? But I didn’t say anything because I didn’t want to be that parent.
2 minutes from the end of the first match, the assistant coach pulled off a player to put in DS. I was sat directly behind them. This kid immediately starts kicking off, saying “you can’t take me off for DS, he’s rubbish, now we’re going to lose and it’ll be all DS fault, it’s not fair” on and on and on. Assistant coach is shushing him (probably because he knows I can hear) and saying “don’t worry, there’s only 2 minutes left, it’s fine. You’ll be playing the whole of the next game anyway.” This kid then got to play the full next match and DS again got to play just 2 minutes at the end - and only then because another child was injured.
I was utterly furious. I didn’t say anything at the time because I was afraid I’d cause a scene but now I wish I had. My poor sweet boy would be heartbroken if he’d overheard, though luckily he didn’t. I don’t feel like it was dealt with properly by the assistant coach but I’m not sure if I’ve missed my chance to address it.
AIBU to be upset? And to think the coach should have handled the situation differently? WIBU to bring it up at this weeks match? Should I approach the assistant or go straight to the coach, or the kids mother directly? I’m not even sure what to say but I feel like I need to stand up for my boy.