David Safir,
Yes, and during setup it is broken down by each part. To use the $59.99 plan as an example
Voice Plan - $39.99, 450 anytime minutes, 5000 night & weekend minutes, unlimited mobile to mobile minutes. The next one is $59.99, and so on for the voice portion.
The next section of that page shows Unlimited Data - $20
Below that there is the text options:
200 - Included in Data plan
1500 - $10
Unlimited $20
If you were to get your daughter an individual plan, with unlimited messages it would be $79.99 for the 450 anytime minute plan and unlimited messages. There would be taxes on that, and the first bill would have a $36 activation, 1 month service, and a prorated month of service. Likewise for the other daughter. However, even if you didn't get unlimited messaging, they would have unlimited mobile to mobile minutes, and could call each other. Perhaps encourage the them to talk instead of text?
The family plans start t $109.99 for 2 iPhone lines, and would be an additional $40 for unlimited messaging for both lines. ($20 each). The difference when compared to the $29.99 family plan option is only $10.01. That would give them 700 shared anytime minutes, unlimited nights and weekends (after 9:00 pm), unlimited mobile to mobile minutes with other AT&T customers. The extra $40 would give the unlimited SMS, for a total of $149.99/month. One thing you can encourage them to do is use email, its included in their data plan for free. You can email other AT&T customers by emailing 10digitphonenumber@txt.att.net. Also AT&T phones allow you to send SMS to email by putting an email address in the to field.
You can see the family plan options on this link and then clicking on the family plan tab:
http://www.apple.com/iphone/easysetup/rateplans.html
Family plan prices on there are for 2 lines, both with iPhones, additional iPhones are $29.99 month ($9.99 for the line as with any cell phone, $20 for the unlimited data + 200 messages). You can add other phones to an iPhone family plan for $9.99/month, or add an iPhone to an existing family plan for $29.99/month.
Hope this helps,
Nathan C.
NOTE: Information in this post is based on rates information provided by AT&T and on the above link. This of course is subject to change by AT&T, and is current as I personally understand it on 1/26/2008. Other than adding corporate plans it hasn't changed, but still it could change.