Here's a few ideas:
Practice twice a week for 1 hour each. Possibly add an optional, small group/individual skills day for pitching/catching/hitting.

Keep practices fun, organized, upbeat and challenging. Be ready to change gears if something isn't working out.

Warm ups: Usually a fun game like glove tag, base races, etc. Kids this young probably don't need much--mostly something fun to get them looking forward to arriving on time, facilitating the love of the game. If you must use a structured warm up, check out:

www.youthfitnessmag.com/warm-up
www.softballperformance.com/products/dynamic-warmup-system.html
www.sbcoachescollege.com/articles/MizzouSoftballDynamicWarm-Up.html
www.girls-softball.com/2008/09/oldnew-school-stretching.html

Here's a couple links:
www.mysoftballcoach.com/coaching-youth-softball.html
www.mysoftballcoach.com/8u-batting-order-strategy.html

Have fun,
Ken