Bella and I tried Magic Milk Paint this past weekend. The end result? Eh. It LOOKS cool, but other than that wasn't very exciting.
The "magic" part is when you touch the food coloring with your toothpick dipped in dish soap - the food coloring "runs away" from it. That IS cool. But lasts about a second and then the thrill is gone. After that, Bella had fun stirring the colors around in milk but it ended up being too messy and wasteful for what it was worth.
My advice would be to skip this project. Save the milk for dinner and the food coloring for cookies.