Since, I am a Child of Deaf Adults (CODA) and hold a certification as an American Sign Language (ASL) interpreter. I thought you might be interested in my choice for teaching your children ASL.
For the elementary crowd keep it fun!
Signing Time - does just that. It keeps it fun while teaching a lot of fundamental vocabulary. Their website has a lot to offer. See the links below. They also have two series. The first series focuses on vocabulary. The second series focuses on putting it all together.
Signing Time Website
Sign of the Week
Search Amazon.com for signing time dvd set
For older children I recommend
ASL University - it is put out by a Deaf ASL teacher that has created this free website to learn ASL. It is spot on and will give any student - young or old a great start in learning ASL.
The other part that you need to truly learn ASL is a community of Deaf people to use it! You will never truly know the language until you can "read" a Deaf person signing to you! This is the challenging part.