Christian advice on magic?

I admire the fact that you're trying to be careful about what your children are exposed too, but if this is going to be a magician at a ten-year-old's birthday party, then I don't think it would be considered a sin. The kind of magic that would be sinful would be dark magic (seances, calling up spirits, etc.). The kind at the birthday party is probably going to be only harmless optical illusions.