If everything goes smoothly in their life, your kids will never have to show their CoC.
However, it is when things go wrong that the certificate can be useful. Passports expire, and there could be a delay in renewing the passport for various reasons, like the backlog that happened a few years ago, or the government denying the passport because they think you owe child support (sometimes they are wrong, and it is a case of mistaken identity or false allegations, but until it is resolved you can't get a passport).
So if you are a derived citizen without a CoC and your passport expires and you are stuck in such a delay when renewing it, you are left with no solid proof of citizenship while you wait.