We are expecting a second child soon. Once my wife's maternity leave is over, should we pay our helper more, since she will have two kids to look after?

My personal opinion would be to adjust the priorities. For instance, some household chores could be done once a week or once every fortnight instead of daily. This will allow her more time to attend to the children. Assess what are your priorities. From what I see, I think time is limited so you'd rather her focus on the more important stuff than to try to do everything. I would only increase the pay if she performs the tasks adequately. So perhaps you can consider giving a bonus at the end of the year instead?