L.B.
What a wonderful opportunity for your family! Definitely take it.
I did a quick search for Shanghai and found a state department site that gives practical info for Americans traveling - ttp://travel.state.gov/travel/cis_pa_tw/cis/cis_1089.html
and another that just talked about what to do in Shanghai (there are lots of these) - http://www.travelchinaguide.com/cityguides/shanghai.htm
Your husband's business associates should also be able to give him ideas of fun and safe things for your and your daughters to do while he is at work. You may also want to see if the hotel can recommend a guide for you while you are there.
The airline may have some information on long flights and traveling with children. Don't forget to check the TSA list of banned items, since it changes daily, to ensure you pack your carry on bags correctly.
Since it is a 20 hour flight you have to assume they will sleep part of that time. Your pediatrician should also have some insight about traveling this long with your girls and what things you should take with you.
I think with enough pre-planning you all should have a wonderful trip.
Have fun and go get your passports!!!!!