OK. There are a number of issues here.
Firstly, you have to define, in positive terms, exactly what behaviour you require from the kids. E.g. if it is sitting still during carpet time activities, you might have a target of "To sit on you bottom, with you legs crossed, looking at the teacher and with your hands in your lap". For some kids, this will be too difficult for them to achiever, so for them you might just have "To sit on your bottom with your legs crossed" etc.
Secondly, you need to have a realistic time frame. For some kids it may be impossible to "be good" for longer than 5 mins, so you need to reflect that in your target. E.g. In the above target about sitting still, for a particularly restless child you might want to add "for 3 mins". In the begining of any program it is important that the child has a good chance of getting the reward on a regular basis. A 4 - 5 year old should get a reward at least every day, a 6 - 7 year old at least every couple of days and a 8 + might be ok if slightly less frequent. However, if you are dealing with children with lots of problems, it will be most effective if you can ensure they get lots of rewards early on. Later you can make the targets harder and so rewards are received less frequently.
Thirdly, children need to be involved in choosing their rewards. This makes it more motivating for them. Get them to draw up a list of possible rewards. Some could be tiny rewards (e.g. sticker, marbles in a jar, smiley face etc), some medium rewards (e.g. chocolate) that is got when child has earned, for example, 5 small rewards, and some big rewards (e.g. 20 mins choosing activity).
Fourthly, you need to think carefully how you are going to monitor this reward system. It will only work if you adhere to it religiously. Realistically you may need to focus only on particular areas of the day e.g. carpet time, play time, co=operative activities etc.
Fifthly, you need to think about how often you are going to reveiw the targets and rewards with the children. About once a week is necessary. If you don't actively involve kids in this, you will find it difficult to move away from "rewards for good behaviour" to "good behaviour for it's own sake", which is surely the ultimate aim.
FWIW, I think sticker charts are overused. If you do use the, you will have to think really, really hard about what you are doing to also move children towards behaving well without the use of tangible rewards.