Lily, I echo what Scarlett said. Also, after the story, let the children act out the lesson. If the lesson is Jesus walking on water, put down a blanket and let one of them pretend to be Jesus while the other sit in the boat (the blanket.) IF the lesson is on Noah's ark, give them each an animal to put into the ark, or better yet, let them BE the animals and have them hop, walk, crawl, or whatever into the "ark." It doesn't have to be elaborate. The "ark" can be under the table, or just an area on the floor.
Change their activity every 5 to 7 minutes. Have a five-minute lesson, 5-minute coloring session, 5-minute snack time, etc. Keep them moving--up for 5 minutes, sitting for 5 minutes.
I've seen teachers try to do everything scriptural, like an hour long "class." Nothing against scripture, but after a 5 minute lesson, there's no sin in letting the kids play with play-dough or dress up for a while. This is preschool, not adult worship. Learning to share while playing with toys is just as scriptural as the pastor preaching an hour to the grown-ups. Keep it at their age level, and you will do fine.