As a rule, children will live up to your expectations. The real question, then, is "What are my expectations?" I've learned quite a bit about expectations in twelve years as the Storylady. When people ask me how I manage to hold thirty toddlers or fifty preschoolers in the palm of my hand for half an hour, I have to say that expectations are the key.
First of all, I try not to expect more out of kids than they are capable of delivering. I don't expect a two year old to sit still and be quiet for more than three or four minutes. I don't expect five year olds to sit still for four books in a row. When we expect more than a child is capable of, we're setting us both up for failure and confrontation. On the other hand, I have high expectations when I know I'm asking for something they are capable of. Two year olds can learn to sit on their bottoms for a short, interactive book. Five year olds can listen quietly to two books in a row. I think they recognize that when I ask them to do something, I believe they can do it, and I expect them to do it, but my attitude is a positive "OK, let's do this now," and not "Here's what I want you to do and you'd better do it, even though I know some of you won't."
I've also learned not to expect to hold a toddler's attention if I break their concentration for more than about thirty seconds. Those songs, rhymes, fingerplays, and stories have to keep coming one right after the other. If I stop for an announcement, it better be between fifteen and thirty seconds tops, or those little munchkins are gone. And it would be completely unreasonable of me to be irritated with them for wandering off. That's where toddlers are at developmentally, and it's MY problem if I take too long between activities, or spend too much time talking to the adults.
The same is true for the preschoolers. If I read three books in a row that don't have anything interactive (like saying repetitive lines together or movement incorporated into the story) then it's my fault if Jacob and Emily are rolling around the carpet or striking up a conversation in the middle of my book. Those expectations are unreasonable for their age. On the other hand, if Jacob and Emily are having trouble settling down to listen to the first book, I make it clear that "Now it's time to sit in one spot and listen quietly," which is reasonable and understandable for a four-year-old.
As always, praise for doing the right thing goes a long way towards getting the children to repeat the good behavior.