EXCLUSIVE

Mackenzie's legal team says, "This is a frivolous lawsuit, which we intend to prove in a court of law. This is yet another example of a production company taking advantage of a young rising star.”

Former "Dance Moms" star Mackenzie Ziegler is two-timing a music producer by bailing on performances and record sessions, and working with another producer on the side ... according to a lawsuit.

Kismet Music is going after 13-year-old Mackenzie and her mom, Melissa Gisoni ... saying they breached their 2014 deal.

In the docs, Kismet says when it signed Mackenzie -- who has more than a million YouTube followers -- she agreed to "deliver technically and commercially satisfactory records." In exchange, Kismet would try to secure a recording contract and distribution deal for her.

The producers say Mackenzie and her mom didn't seem to have any issues with the contract when they were cashing checks associated with music released under the deal -- but in 2016 something changed. According to the suit, Mackenzie started skipping recording sessions and meetings -- and when Kismet tried to contact her, at least 14 times ... radio silence.

Maddie Ziegler's kid sister is also working with another producer, a clear violation of her exclusivity deal with Kismet ... according to the suit.