I've just sat here with my iPad and iPhone and face timed myself! When I opened FaceTime on the iPad I "rang" my iPhone from my contacts list. Not sure how you dial directly if the number isn't saved in your contacts though.
Add her e-mail address to her contact details on your iPhone. When you open the contact, press FaceTime. It will ask if you want to use a phone number or e-mail address. Click on the e-mail address and it should FaceTime your mum's iPad.
Ok, that makes sense... I think. I have to go and check that her iPad has contacts - she hasn't really touched it yet in case it explodes or summat . You know, newfangled things, they can be unpredictable and dangerous, just like wild animals...
Your mum will also have to make sure she has FaceTime on on her lap top, and to pit the email address she wants to be contacted on in the Face Time Settings.
An iPad does have a 'phone' number if it has a sim card in, but you can't call it. Though I tried sending it a message from my iPhone which apparently went, but hasn't turned up. Then I sent one to my iPhone from the iPad, which arrived and I was able to reply from my phone to that message.