Here I go again recommending foam earplugs. . .

Surely I'm going to get tagged by the Tag Fairy as "The Earplug Lady".
That said. . . I love foam earplugs! For whatever reason, the rides and attractions at the Disney theme parks are extremely LOUD. You can purchase a pack of foam earplugs at your local Mart in the Health & Beauty Aids section. Or if you know someone who works in a manufacturing plant, they can probably get some there. You'll be better off with the ones shaped like bullets than marshmallows, though, since you're working with an infant with tiny ears. Basically, you roll them between your fingers to make them long and skinny, compressing the foam. Gently pull on the ear to make the ear canal a little straighter, insert a tiny bit of the earplug and hold it there while it decompresses and VOILA!
You aren't sticking something way down in the ear canal, so there isn't any danger of damaging her ear innards. Plus it's squishy foam, so it conforms to wherever it's put.
DS5 is a kid who doesn't like darkness or loud noises (nothing developmental, he just likes quiet, bright places). He spends about 60% of his time in the Disney parks with his earplugs in! That removes the aural sensory overload so all he has to manage is the visuals.
So there it is, yet another "plug" (

) for earplugs! BTW, DH laughed at me when I bought a case of 100 earplugs on strings! But it was $15 for a dozen and $19 for 100!
Happy planning!