Nutella
This was something I did for April's Fools Day last year, and it was great success. I filled our empty toothpaste tube with Nutella and left it in its usual place by the sink. Needless to say, the little ones were delighted when they discovered the special toothpaste. Please, only do this experiment if your children are old enough to understand why they can't brush their teeth with Nutella every day ;)