Brightness can be turned down by going into 'Settings' then 'Display and Brightness', turning off 'auto brightness' and then adjusting the dial to low. Please also check you are not using the iPhone charger by accident, which is only 5W, this is the lowest source of power and will drain the battery quickly. A 12W adapter is supplied with the iPad Pro.
Alternatively, a higher watt adapter can be purchased (must be authentic Apple), between 30W to 61W.
NOTE: If these bigger adapters are used, you need to check if a USB-C to lightening cable is needed (if it's purchased as is, the customer won’t be able connect direct into the iPad).