Your first practice if vital. Be prepared - first impressions are important, especially with 4-6 year old kids. A good way to begin is by making introductions. Keep in mind, some kids might be nervous, and making sure you know everyone's name could help them relax. Also, at this age you will have boys and girls of all sizes and ability. Start with some very basic exercises, such as running the bases. This will help release some of the nervous energy while giving you an opportunity to see each child's athleticism.
Solicit help, and lots of it. Hopefully you will have some parents who want to get involved and help on the field. This will allow you to split the kids into smaller groups of 4 or 5. One group can practice hitting while another practices hitting. Ideally you as the coach will oversee all of the drills and rotate from group to group. Your league should have a policy for doing background checks on all coaches. Make sure you and your coaches comply with the league rules.
Spend your first several practices working on the fundamentals. Remember, many of these children will be playing baseball for the very first time. Nothing is too basic for this age group. Start with tennis balls or soft baseballs so that the kids don't get hurt. As time goes by introduce the official league balls you will use during games. For some great drills, visit the resource listed at the end of this article.
Attention span at this age is a big factor. Keep the practices short (45 min. to 1 hour) and fun. Too much standing around and waiting will cause your players to lose interest. Close each practice with a contest such as racing around the bases. This will keep them interested and give you a motivational tool should you need it.
As you approach the first game, review the rules. Do a lot of base running drills with base coaches in place simulating real game conditions. This will save you a lot of outs as the season continues. If possible, set up a scrimmage against another team.
Moms can help too. If they don't want to help coach, some might be interested in serving as administrative assistants. They can distribute schedules, notify of rain outs, get uniforms, etc.