Here are some resources that may be helpful:
http://www.cal.org/resources/faqs/rgos/asl.html
http://en.wikipedia.org/wiki/Manually_Coded_English
http://www.drury.edu/multinl/story.cfm?ID=9901&NLID=166
http://www.websters-online-dictionary.org/de/deaf.html
Conatct: Galludet University and California State University at Northridge for more information on the linguistic aspects of ASL, and specifics on how to advocate for your children's right to communicate in ASL if that is their preference and aptitude.
Forbidden Signs: American Culture and the Campaign
Against Sign Language.
D.C. Baynton
Chicago: University of Chicago Press, 1996.
Phone: 773-702-7700
ISBN 0-226-03963-3.
I also suggest you Google "American Sign Language vs. oralism" and similar topics to find resources to help you. At worst, I think you can show that oralism and ASL are about equal in terms of language development - they are just different.
You will want to focus on how total oralism is not a goal achieveable by all deaf people, and that it alienates deaf from their own community while at the same time failing to adequately integrate them into a fully hearing community. You will want to reasearch how often oralism fails - though there are notable success stories - it's not yet a temendously effective way of dealing with deafness. You may want to consider having the chidren ALSO learn SEE Sign (Signing Exact English) as a way to develop more formal academic written English skills - not that it is commonly used in the deaf community, but it will show courts you are taking language development and communications skills seriously if you are faced with people who think ASL is somehow "lazy" or not a fully developed language of its own. ASL is recognized by many experts in linguistics as a fully developed language, similar to English, but unique and freestanding as a language.
The user on this forum who uses screen name "Nesmuth" is a deaf advocay activist. You may want to private message him. he may have more specific suggestions.
Above all, understand their mother's concern - there is much debate on this topic and the jury is still out. Many "hearies" do not understand that ASL is a distinct, unique, fully expressive language. Unfortunately, many people just see it as "dumb people English". ASL reminds me a great deal of Chinese and Japanese where "pictures" represent complex concepts and ideas are not broken down into letters and words the same way as they would be in English. You certainly wouldn't say Japanese or Chinese aren't languages. If you can reform her opinion about ASL - maybe you can help her understand. Hopefully it is legitimate concern on her part and not just a ploy to gain custody.
