how much is it to add your children's x 2 on your mortgage? And if I did do this would it help them on their way to get there own mortgage as they are not on a credit rating system as yet??

If you add your children to your mortgage you're really not doing them any favors, especially if they don't have any credit rating yet. They may have trouble getting loans in the future if they're already listed as being a borrower on your mortgage.

If they want to start developing good credit then they should apply for a department store credit card, buy a few things and pay the bills early and in full. That will get them started on a good credit rating. Once they've developed a good credit history they can move on to other credit, like credit cards, car loans and mortgages of their own.