if I temporal breach a solo unit at a battlefield with a hidden card do they have to reveal the hidden card?

No. Temporal Breach banishes the unit and then its owner plays it back to the same battlefield as part of the same resolution, so your opponent never loses control of the battlefield and the hidden card stays facedown.

Control loss (and the hidden-card trash that comes with it) only happens in a Cleanup when a battlefield has no units and no Showdown/Combat is ongoing (Rules 323.6–323.7), and control can't be lost while there's an item on the chain. Even during the brief window while the replayed unit is pending on the chain, control is locked. They may choose to reveal the hidden card as a reaction before Temporal Breach resolves if they want — hidden cards gain Reaction while facedown — but nothing forces them to.