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?

It will be good if you can discuss with your helper or her agency if she is agreeable with the amount of increment or if she is okay to continue her service. I have a few friends paying their helper slightly more when their second child came. One of my friend's helper decided to terminate her service when their second child came as she said she cannot cope with all the duties i.e. household chores, cooking, looking after 2 children.