It's often said that many people can't or won't complain about bad service, or things going wrong at school: perhaps because we're too polite, don't want to cause trouble, we feel sorry for the person who has to take the complaint but can't do anything about it, and so on. There are loads of threads on MN "AIBU to raise this with the school", "AIBU to complain about builders wolf-whistling", "AIBU to complain about upselling in WHSmith" because we hesitate about if it's the right thing to do, rather than just doing it.
As a young adult, I found complaining very difficult: I'd be expecting a reply of "oh stop moaning" that children so often hear when they are growing up, I found it very hard to separate childish moaning from legitimate complaining. I often lived with the problem, instead of complaining. A couple of times I witnessed my mum raising hell in shops when she was overcharged, or boldly telling off teenagers on a train who were stampeding all over the seats, using phrases such as "I shall not hesitate to get the guard" in her best teacher voice. These didn't turn me into a confident complainer, in fact I think it made me hesitate all the more, feeling that you had to do it like that!
Nowadays I do complain if I have to, but it doesn't give me any pleasure to do so. If I get a discount for having complained, I almost feel that I've cheated the company, not the other way round! On the other hand, you hear of people who take a businesslike approach to complaining: after paying for anything, from a train journey to a holiday, they make sure to complain about something, so that they might get a voucher.
There are occasions where speaking out can actually be disastrous: re the teenagers on the train, some people might reply now "she shouldn't have done it, they have knives now", and if speaking out is referred to as "whistleblowing" or "grassing", that's usually considered bad.
Do you actively teach your children about when it's right or wrong to complain? It's actually quite complicated knowing when to speak out, and when not to, there have been many threads about individual cases. There was a thread some time ago about a teenage boy in class messing about with a condom, which the teacher totally ignored (AIBU to complain about teacher doing nothing?). There was another about if a child is playing up, and one of the well-behaved ones says "Miss, so and so is messing about" it's actually more disruptive, but it can be hard for a child to understand that.