Some kids are really sensitive to loud noises. So what's a parent to do. Well there are nose headphones and ear plugs but what if the sensitivity is super high.
Well time to go to the web and find out how the pros do it. First off how are these things measured. They use NRR or Noise reduction rating. The math is a little obtuse. Basically every 3dB is half the energy or level.
So if you are trying to protect Again fireworks. They sounds to me like a jet plane takeoff  close or 120dB and want to get to say a quiet room which is 40dB then you need over 80dB of reduction.
Now you would think that an NRR of 33 would result in 33dB reduction but it's much less. The actual reduction is (NRR-7)/2 or it allows 33-7=26 and then divide by 2 to get 13dB. So that takes 120-13=107dB
So the next step is to use earplugs with earmuffs. Then the math is the (maximum NRR-7)/2 + 5dB. This doesn't seem quite right because it doesn't factor the earplugs but it's roughly right. So in the above example wearing earplugs gets you to 102dB not super close to 40dB.
Now in terms of choices here are the big ones

1. Active ear muffs. These use AA batteries and you can hear normally but when there is a big noise it actively cancels them. Kind of like sometimes Boss Comfort phones.
2. Passive ear muffs. Just lots of padding. The main thing is fit for a small head.
3. Earplugs. These are less comfortable but they are equally effective.

The shooting review and wearplug has a good list as well as a concert tuned list.

1. Howard Leigh R-01902. This provides an NRR of 30 and major noise reduction at 82dB. Not quite sure what that is but sounds like a good idea.
2. Clear Armor 141001. This has a high NRR of 34

Earplugs are actually incredibly effective given how small they are

1. Macks shooters ear seals. 27NRR
2. Ear defense high fidelity earplugs. 27 NRR
3. LiveMusic Hearsafe. With the white filters you net 29 NRR and they also have a lower one. $18 at Amazon 4. Decibullz Custom molded. These are 31 NRR and mold by putting them in water. Good ratings at Amazon.$24.

Then there are baby oriented products with about 20dB

1. Pro Ears Revo. These are for teens with 25 NRR but they fit smaller heads. $42 for passive and$82 for active but you can get from Amazon for \$33