Depend on the kid
A friend of mine researched that topic before taking his son to an airshow few years ago. Pretty much any type of earplug is out of question, only ear muffs, and those have to be either highly adjustable, or pre-fitted, as if they're to tight they won't stay on for to long
Kids prefer jet roar to anything that makes them uncomfortable.
It is also a good idea to have it tied to part of clothing. Friend of mine, being a firefighter, did a months long study on those, bought few pares... and as it turned out his wife was the one who needed ear protection, the kid slept through MiG-29 solo... and trust me, there is nothing louder than two RD-33 on full afterburner, the ground shakes, your stomach switch places with heart, and you can't communicate anything... and the 3 year old was sleeping peacefully
