If I have a hidden card like Keeper of the Mask and I have a temporary card like Sprite Token and in my beginning phase before scoring on the temporary trigger I reveal my hidden card Keeper of the Mask will the Keeper of the Mask die on that beginning phase?

No, Keeper of Masks will not die during that Beginning Phase.

According to RiftJudge FAQ #9406, when you respond to the Temporary trigger of an existing unit (like your Sprite) by playing a hidden card (like Keeper of Masks) from its facedown state, the new unit and its generated tokens remain on the battlefield for the rest of your turn.

Reasoning

1. Trigger Timing: The Temporary keyword is a triggered ability that activates at the start of your Beginning Phase, before scoring (Rule 816.1.b).
2. Chain Resolution: When you react to the Sprite’s Temporary trigger by flipping your hidden Keeper of Masks, the Keeper of Masks enters the battlefield, and its "When you play me" ability is placed on the chain.
3. LIFO Resolution: Because the chain resolves in Last-In, First-Out (LIFO) order, the Keeper of Masks' ability (which creates the Reflection tokens) resolves before the original Sprite's Temporary trigger finishes resolving.
4. Trigger Window: The Temporary trigger check for the Beginning Phase happens only once per phase. Because your Keeper of Masks and its tokens enter the battlefield after the initial trigger check for the phase has already occurred, they are not subject to the Temporary kill trigger until the next time your Beginning Phase starts (Rule 340, RiftJudge FAQ #9406).

Sequence of Events

1. Beginning Phase starts: The Temporary trigger for your Sprite is placed on the chain.
2. Reaction: You play your hidden Keeper of Masks in response to the Sprite's trigger.
3. Resolution:
   • Keeper of Masks enters the battlefield.
   • Keeper of Masks' "When you play me" ability triggers and is placed on the chain.
   • Keeper of Masks' ability resolves, creating the Reflection tokens.
   • The original Sprite's Temporary trigger resolves, and the Sprite is killed.
4. Result: The Keeper of Masks and the new Reflection tokens remain on the battlefield, as the trigger window for this turn has passed.