Getting started in childrens shows is really not hard. There has been alot of good advice posted, but also some that is not so good. We all have our opions. My opinion is this....Just do it. A Magician is never going to be prepared unless he/she actually does it. There is no way to know what prepared is until you do a few shows, and then realize what needs to be worked on, what works, and what doesn't. It is all trial and error. You can spend years "thinking" about it, but never know until you do it. Another thing is "Yes" the magic should be exciting, but at the same time, so does the magician. YOU make the magic happen.
Booking a free show every now and then is not bad to do, but at the same time people will take advantage of you if they realize that you are available for FREE shows. Not good. You can however use them as tax write-offs as long as you have them recorded via video for the IRS in case they ask (Charity donations).
Plain and simple, you just have to go out there are do the job, and see what YOU need to work on. You'd be amazed at what you see on video that can make all the difference in the world.
As for tricks, that depends on the age of the group you are performing for. Young kids won't understand some things, and you have to gear to their mentality. Also, kids have a 20 minute MAX attention span, and then they are ready to jump up and do something else, so keep that in mind also.
Someone mentioned in an earlier post about ending with a BANG. YES YES YES!! I always end my shows with a huge bang, and leaves the audiences excited. I start with a bang, slow down, and then a one -two punch and knock them out. Here is an example of one my recent shows http://www.magicsax.com/Videos/060305.mov
Another thing is - GO IN WITH CONFIDENCE!! - you would be surprised at what people can pick up by looking at you. Be confident (NOT COCKY!) about your show, and you will portray that onto the spectators. If you think you are good, then they will think you are good. If you show weakness, then well, the result is weakness. KNOW WHAT YOU ARE DOING AND PLAY THAT ROLE! - you'll succeed 100% of the time.
Hope that helps a little *smile*