I am actually curious about that, performing for kids around 7-8 years old is one thing but 4 or 5 years? That's definitely not my alley, and thats why I have declined some jobs involving that. What is your opinion? Should you stick to the things that you feel comfortable doing or risk it for the job?
Kids shows are very different that typical "street" magic. Although you may be able to entertain a group of kids for 5 minutes with a card routine, the whole show cannot be cards. Most kids shows need to be around a half an hour to 45 minutes. Also, the presentation is more like close up or parlor than street. You most likely will perform using a table or stand.
You need a variety of effects such as sponge balls, cups & balls, linking rings, rope tricks and other small stage illusions (zombie ball, magic milk picture, magic candy maker, vanishing bandanna, etc.).
Also, kids shows require a lot more patter and interaction. Also, you have to have a good sense of humor and be able to be silly. The best kids shows have humor that both the parents and kids enjoy (think of how the movie Shrek appeals to everyone from 5 to 75). You also need to have very good audience control skills and lots of confidence. Kids love to shout out during performances, everything from "I know that trick" (which is really funny when all you've done is asked someone to pick a card) to shouting out the name of the card that you are not supposed to know. Also, if you make a mistake, the kids will call you out on it.
Also, as you recognize, there is a real difference between kids in the 3 to 4 range, the 5 to 6 range, the 7 to 10 range and the 12 to 14 range.
My advice is don't do a paid kids show without a lot of preparation and practice. If people are paying you, they expect a professional with a perfect presentation. There really is no room for error... otherwise, you have one unhappy customer.
If you are interested in children's magic, check out "Seriously Silly" by David Kaye (there is a book and a DVD) and "Kidding Around" by Chris Capehart (DVD).