WhatsApp now widely testing advanced privacy control; which means you can hide your last seen from mom.

Finally, WhatsApp enabled advanced privacy controls for more testers; which means now you can hide the last seen status for specific people and more.

The advanced privacy controls will give you My contacts except… options to your Last seen, About and Profile photo menus where you will have more control over that.

PS: This feature is currently in testing and will be available for stable users soon.