They should be fine as the heat loss from the tank is gradual, most shrimp/fish can withstand gradual temperature shifts, sudden/rapid temperature shifts are more of a concern. The bags with warm water should be more than sufficient, just stick to the bags don't try to add warm water directly as this can change the water temperature a few degrees if you don't match the water temperature and gradually go up in heat.
Depending on how long the power is off and how low the temperature gets you may want to turn the light on and turn the heater down to match the temp and gradually increase it back to normal. Good luck and I hope all is well once the powers back.
