Invest in a higher-quality, more durable brand of playing cards, such as those used in casinos, if you intend to perform professionally. While more inexpensive decks are prone to split and crease, casino and poker decks are made with a protective plastic coating. This also makes it easier to perform impressive actions with them, such as fanning the cards.
Know your audience and adapt your performance to engage the children's attention, but do not "play down" to them. As renowned lecturer and experience children's performer Joshua Jay remarked, "There's a common misconception that kids are easier to fool than adults. My experience shows quite the opposite. From a child's perspective, the world is a place filled with magic ... by comparison, the effects performed by professional magicians can seem unremarkable." Far more important than what effect you perform is how you perform it.
Study the classical books that magicians have been taught from for nearly a century. Performers have learned from trusted volumes such as Hugard and Braue's "Royal Road to Card Magic" and the more advanced "The Expert at the Card Table" by S.W. Erdnase for years. Doing likewise will empower with many sleights and techniques which you may then adapt to child audiences and to fit your style.
Learn and practice the basic techniques of card handling. While the individual effects you perform for child audiences will vary, the basics will oftentimes remain constant: If you are right-handed, hold the deck in a dealer's grip (also called a mechanic's grip) in your left hand. If you are left-handed, reverse these directions accordingly. Your thumb should lie alongside the left edge of the deck with your index finger curled against the end pointing away from you, and your remaining fingers cradling the edge facing to your right.
Deal from this dealer's grip by pressing down with your left thumb onto the corner of the topmost card, then sliding to the right. Take the card with the fingers of the right hand. In magic, this is often called "thumbing off" the top card.
Involve the children and provide opportunities for them to participate as much as possible when creating and performing tricks. Joshua Jay goes so far as to say that "the secret to entertaining children is interaction."
For example, have a child volunteer be the one to shuffle the deck at the start of the trick. As he returns the deck to you, discreetly glance at the bottom card -- called a "key card" in magic. Have him cut to a random place in the deck, note the card there and complete the cut, placing your key card on top of his. Have him cut a few more times to mix the deck as opposed to shuffling so as not to separate the two. Reveal the card in a way that will be entertaining to children, such as dealing the cards face up, one at a time. Once you have dealt both the key card and his selection following it, pause and claim that the next card you turn over will be his. As the children laugh, believing you have erred, reach down to the table and turn over their selection face down, thus keeping your word.