Depends on if you want to 'get into it' with her or not. If you want to take the high road a simple "Thanks for the thoughts Mom, but me and my family are perfectly happy and content right how we are". At least you acknowlegde the thought/concern, however warped it might be.
On the other hand, there is PLENTY of material on this site and others that you could launch into a full blown attack on the WTBTS if you wanted Maybe start with asking her why "Jehovah" would want you to teach your children about an org who was a part of the 'wild beast'? See threads related to WTBTS being a UN NGO for detail... there's plenty more.
Good luck either way - I'm waiting for the day when my parents hit me with one of these guilt letters too ...