The Wand of Orcus is the only published magic item that can can be used to raise and control a large number of undead (as the question originally asked).
If this 13-year-old somehow got her hands on the Wand of Orcus, she would be able to use its Call Undead feature:
While you are holding the wand, you can use an action to conjure
skeletons and zombies, calling forth as many of them as you can divide
500 hit points among, each undead having average hit points. The
undead magically rise up from the ground or otherwise form in
unoccupied spaces within 300 feet of you and obey your commands until
they are destroyed or until dawn of the next day, when they collapse
into inanimate piles of bones and rotting corpses. Once you use this
property of the wand, you can't use it again until the next dawn.
The Wand of Orcus is an artifact, the most powerful type of magic item in the game. It is normally possessed by the demon lord Orcus, and it would only find its way to the girl's hands if Orcus willed it. The wand has many other features and abilities, which are detailed in its entry in the Dungeon Master's Guide.
More common ways to create and control undead
The usual ways a player character could raise and control undead are via the spells animate dead (3rd-level) and create undead (6th-level). The number and type of undead created varies based on spell and spell slot used.
A low-level character who has obtained a spell scroll for either of these two spells, has a chance of casting the spell from the scroll, even without being normally able to cast 3rd or 6th level spells, as long as they are of a class whose spell list includes the spell in question (animate dead is on the cleric and wizard spell lists, while create undead is on the cleric, wizard and warlock spell lists).
- Casting animate dead from a spell scroll will create one skeleton
or zombie. Casting it at higher levels could create more skeletons or
zombies, but this is not possible from a spell scroll.
- Casting create undead from a spell scroll will create up to three
ghouls. Casting it at higher levels will create more ghouls or
more powerful types of undead, but again, this is not possible from a
spell scroll.
Aside from spell scrolls, I was able to find two other published magic items that can cast animate dead, but none that can cast create undead.
- In addition to its more powerful Call Undead feature, one charge of
the aforementioned Wand of Orcus can be used to cast animate
dead.
- In addition, in the adventure "The Sunless Citadel," contained
in Tales from the Yawning Portal, there is a magic whistle
called night caller, an uncommon magic item that can be used
to cast animate dead once every seven days.
Both of these items will only cast animate dead at its lowest possible level, though, meaning that only one skeleton or zombie will be created.
(Some of these links will only work for those who have the appropriate access in D&D Beyond.)
Addendum: The question as currently worded merely asks for an item that can be used to summon monsters, rather than to raise an undead army, as it did at the time of my answer above.
A bag of tricks may fit what you are looking for. The fact the bag contains "small, fuzzy objects" fits with the idea of a kid finding it, playing with it and inadvertently loosing dangerous creatures.
As written, the bag can only produce three beasts per day, and they are friendly to the bag's user and his or her companions, and they disappear at the next dawn. But if you intend that only the NPC uses it, then feel free to tweak the bag's properties to better fit what you have in mind.
