Okay there was fear, but the Saxon elector would not have actually gone through with it. Saxony's army was small and poorly trained. Unlike Prussia, Saxony was not hypermilitarized and simply had no chance of success.
The reason there was fear was because the Saxon Elector was married to the eldest daughter of Joseph I. She legally had the best claim to the Hapsburg Realm. Silesia being the richest province of the Hapsburg realm made it the most likely for them to seize. Also Saxony was a Protestant province and Saxony was Protestant as well, the only reason the elector was Catholic was because he converted to become the King of the Polish-Lithuanian Commonwealth. Therefore if Saxony was going to take anything, Silesia seemed the most likely target. But as I have already explained Saxony had no chance of independently seizing Austria.
If you want Saxony to gain something from the War of the Austrian Succession then there is an alternative. During the war the Saxons originally started on France and Prussia's side. However they switched sides after the war began to militarily favor Austria. So instead of switching sides have Saxony fight on. Then have the Bohemian and Moravian campaigns coordinate better and play out better. The result being the Elector of Bavaria marches into Vienna and the Elector of Saxony secures Moravia. Then in the peace treaty have Saxony be awarded Moravia and something called the Silesian Corridor. Which was a recommendation for Saxony to gain a small strip of Silesia to connect Saxony and Moravia. And the thing is Prussia was in favor of it, because it provided them a buffer against Austria. And voila you have Saxony gaining Moravia and then some, now they ave connected the Polish-Lithuanian Commonwealth with their German lands.
But if you are determined to have Saxony invade Silesia there are two ways it plays out. Saxony invades and then struggles to conquer Silesia, before the Austrian army arrives and defeats the Saxons. Then the Austrians overrun Saxony. Or Saxony invades and struggles to conquer Silesia. Then Maria Theresa fearing her own army's capabilities accepts Frederick's offer to militarily defend Silesia for her. He invades and overruns Saxony and curbstomps the Saxon army. Then he demands recompense in the form of Julich and Berg, a third of Silesia, or gains in Saxony. Maria Theresa either accepts or she denies and begins a war with Prussia.