What's the difference between baking soda & baking powder?
Baking powder is baking soda plus a powdered acid. BP produces CO2 when wet and heated, BS produces CO2 when combined with an acid such as buttermilk, yogurt, vinegar. Fruit puree like banana and pumpkin also seem to be acid enough to interact with BS, because it is often part of those recipes.
My guess is that the cake will be ok. If I made the mistake before adding the liquid ingredients, I probably would add 1/2t of BS. I've seen a lot of recipes that call for 1/2t of bs, and 1 or 2 t of bp. If the batter is already mixed, I wouldn't try any correction.