1. Do this in advance. Get a ripe banana and a toothpick. Prick the Banana with the toothpick and slice it in half without damaging the peel. The hole left by the tooth pick should be invisible. Ofcourse the children don't know this part.
Now you are ready to face the children. Hold the banana with the tips of your fingers (left and right hand) so that the middle of the banana is before your eyes. Tell the children that you can slice the banana by nodding your head (turn your face up then down in one swing while still holding the banana). They will not believe you. After uttering a magic word, peel the banana and Hey presto. It is already sliced. With practice you can slice twice or thrice for a long banana. Make sure you have several PREPARED bananas so that you can do an encore if the children demand. Have some unprepared bananas for the children to try and fail.
2. Get two plastic cups. Cut the bottom away from one of them then insert it into the good plastic cup (the one still with a bottom).
Now tell the children you want to do a trick but you don't remember very well how it is done. Put the cup(s) inside a hat where the children cannot see inside. Tell the children that you have changed your mind, then remove the cup (only the one without a bottom). They should not know that a cup has remained in the hat. Pretend that you want to pour want in the cup (which they can see on the table - which has not bottom). As you talk aimlessly, pour the water in the hat by mistake (of course the water will go in the cup in the hat). The children will really laugh at your absent mindedness. Show regret for pouring water in the hat instead of in the cup on the table. But since you are a magician, you can solve that problem. Utter some magic words, put the bottomless cup back in the hat (in the good cup which now has water). Utter more magic words. Remove the cup(s) and place on the table. Turn the hat upside down to show that it has no water because of your magic. Now HEY PRESTO - pour the water from the (twin) cup to much applause from the chidren.
I hope you have fun with these two simple tricks. A little practice will turn you into a flawless magician and hopefully you can add a few more to your repertoire