Learn Russian: Мы с мамой приготовили кое-что вкусное.

Кое-что implies that the speaker knows exactly what they made, but they are keeping it a secret or creating a little suspense. If they used что-то, it would usually mean "something (but I don't know or don't care what)", which wouldn't make sense since they just cooked it themselves!

Why is the adjective вкусное in the neuter form?

It describes the pronoun кое-что. All pronouns based on the word что (like что-то, ничто, and кое-что) are grammatically neuter. Because of this, any adjectives attached to them must take the neuter ending, like -ое in this sentence.

Why does the sentence start with Мы с мамой to mean "Mom and I"? Doesn't that literally translate to "We with mom"?

Literally, yes, but this is the standard Russian idiom for saying "Mom and I". Russian uses Мы с plus the instrumental case to pair the speaker with another person. You could say Я и мама, but Мы с мамой sounds much more natural and is what native speakers usually use.

Does the verb приготовили take the plural form because of Мы с мамой?

Exactly! Since the subject is grammatically Мы (we), the past tense verb must take the plural ending -ли. Even though the English translation is "Mom and I", the Russian sentence revolves around that plural "we".