Great idea, and the lighting is good. I might have zoomed in and cropped it more tightly to focus more on the marshmallows, and less on the plate, but I like the concise, uncluttered nature of it just the way it is.
Those are some sad-looking marshmallows... it looks like some of them were sat upon :) As far as the technical aspects of the picture, if you could have adjusted the lighting to avoid the glare on the top left and the blue shadow on the right it would have been better.
My daughter would eat these up faster than I could take the picture. As for the picture, the background looks more grey than white, the picture looks grainy too, but that could also just be the plate surface affecting it.