I have no business giving advice to any magician who performs for kids so I will keep this brief. Go with sponge balls. I was put on the spot and did the most basic routine in this video (I happened to have a couple sponge balls in my desk). I love the fact that I am winning with a trick I learned in my first magic book from twenty years ago - Mark Wilson's course. It is a classic for a reason. (Side note: I recommend the Jay Noblezada Penguin video on the subject for anyone who is new to this; he obviously didn’t invent the trick he but he offers a lot IMHO).

https://www.youtube.com/watch?v=R0ZTC2GTxMw

Kids and sponge balls go together like peanut butter and jelly. I always carry sponge balls with me whenever I leave the house, since you never know when the opportunity to brighten someone's day might present itself, particularly a child's. A simple sponge routine often elicits the biggest audience response when I perform. Thanks for sharing your office magic.....reminds me of the many times I did something similar when I was in the corporate world.