You might decide if you want a quick opening or a full routine. There are good cases to be made for both. A quick opening says "magic" to kids right away. A full routine may engage them more from the start. The important thing is that you get their full attention before the first trick starts.
One way to do that is to poke your head out from behind some hiding place when you are introduced. Say, "Hello, boys and girls! When I come out, I will say, 'Good evening,' and I want you to say back to me, 'Good evening.' OK?" Then, withdraw for a couple of seconds, and stride onto the performacne area confidently, and bellow, "Good afternoon!"
One of two things will happen:
1. Kids will weakly respond. If so, say, "No, no, we need it louder so that I can hear you! Try again!" Go back to your hiding area, and stride out again. The second time, they will ROAR!
2. Kids will roar "Good evening!" at you!
When they do roar, stagger backwards as if they "blew you off the stage." Either way, you have welded them together into an audience, gotten their full attention, made them a part of the show already, and let them blow off some steam of anticipation. They are now ready to watch your magic show.
Keep it short. Twenty minutes of well-done magic beats 40 minutes of weak stuff every time. Stay away from heavy manipulation (kids see too much of adults with skills that kids don't have yet) or anything that might seriously fail. Stick with easy stuff: Sponge Balls, Afghan Bands, Cut and Restored Rope, Mis-made Flag, Dove Pan, Square Circle, etc. Card tricks don't play well for kids; however, one good card trick, if you must do one, is the Card Through Hank Penetration (in most good magic books for beginners, including the Mark Wilson Complete Course In Magic).
Remember to involve the kids along the way:
1. Perhaps every other trick can use a kid assistant.
2. Have the other kids shout out the assistant's name, and applaud for him or her after the trick as he is seated.
3. Remember to come up with a magic word (can be as ridiculous as you want, or tie into the library theme) for the kids to shout, and don't let the tricks work until the word is shouted. Sometimes you can "forget" to have them say the word, and the magic will not work until they do.
One good running gag is to mis-name someone and stick with it. "Hello, George, what is your name?" gets a giggle every time. When the kid says, "Bobby," you say, "This is George-who-calls-himself-Bobby," (or Mary-who-calls-herself-Katie). Then use this gag for each kid you use in the show, using a different made-up name for each one. If you actually hit someone's name correctly, pull a Pee-Wee Herman: "I knew that!"
Getting back to that opening, try this: Rip open an end seam of a change purse enough to poke in the end of a magic wand at the bottom of the purse. Stuff a 5 foot length of rope into the purse, and close it. Run the rest of the wand up your left sleeve, and hold the purse in your left hand during the shouted opening above. Hold up the purse, and announce that you brought a magic trick in it. Open the purse, and take out the rope, laying it on your table or draping it around your neck and shoulders. Reach back into the purse, and slowly withdraw the 12"-14" wand from that tiny change purse. Look at it in amazement yourself, grin at the kids, lay the wand on the table, and go into cut and restored rope. Use the magic wand to restore the rope (or tap the rope to magically remove the knot in plain sight, your choice!). There you have it: a quicky opener in the wand from purse that says "magic" immediately, and then a full routine to engage the kids while using the magically-produced wand.
Do a Google or Fazzle search for the "Collected Wisdom of Magic Talk" site and read the whole kids section for more ideas. I do hope this has been some help to you. Good luck on your first show, and remember that the kids will be having fun if you look like you are having fun!