I think Alpine also make earplugs for kids. They are, like some other mentioned, quieting the sound without losing too many details.
It's great that you care for your child's hearing. Now that the event is over, you can't do anything about it anymore, and your kid seems fine; it looks like everything is ok now.
But if you plan on more events, you can get some for next time, because cumulative damage can happen. In my experience it also helps to have breaks in between loud events, if possible. So if you know that fireworks are happening because of a holiday or whatever, you can use ear defenders / ear plugs then, too.
I'd like to add to the argument mentioned on this thread, that kids will tell if it's too loud: kids might also think something is ok because their parent said it's ok or doesn't offer ear protection / ear defenders. This could be a reason why they don't say anything.
And often you only realize how loud something was when you get home after the event, not immediatly.
It also doesn't matter if other people are fine and went to hundreds of concerts. That's great for them but you won't know that beforehand, how many loud gigs / events are one too many. Everybody is different.
The better quality earplugs - not foam - are usually less than 20 euros and tinnitus and hearing loss can be massively annoying (and hearing aids could be very expensive in the future). Hearing loss can also influence what kind of job someone can do.