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: