Children's book?

We’re looking for a children’s book (pre-teen) we saw in a bookstore, but can’t remember the name. The protagonists are a boy and a girl who are totally unlike each other.

There’s something about a package that arrives, which they have to open, but they have to find five keys. If they don’t find the five keys, then what is in the package will be destroyed with some dire consequences.

Does this plot sound familiar to anyone?
I appreciate any leads you can give me.