The Boxes — William Sleator

How could it hurt to take just one peek?

When Uncle Marco goes on one of his mysterious trips, he leaves Annie in charge of two exotic, sealed boxes on one condition: she must not open either one while he is away. But of course she is tempted . . . and soon she has unleashed the unspeakable. The creatures inside the box are crablike and grotesque. And they possess a power Annie could never have imagined. . . .