Can I manage the restrictions for my system on two different smart devices?
The restrictions can only be managed by one Nintendo Account. However, you can link the same Nintendo Account to the application on multiple smart devices. Then, you can manage the restrictions through each device that the Nintendo Account is linked to.
@wigglegiggle yeah, you need to sign into the Parental Control App with their mum's login details and the console should already be linked. It's a bit short-sighted by Nintendo that you can't have the parental controls linked to anyone in your Family group that is marked as a parent/guardian.