Translation:The child only eats powdered sugar.
You can replace alleen maar by enkel, which is fine, but used less common. I think it's used more in Belgium. You can also stick to alleen (leave out maar) this sounds a bit less 'nice' to my native ears, also I think it has less emphasis on the only part.
You cannot use maar in this sentence, it would change the meaning, because this either means only in the meaning of but, e.g. Ik vind eieren lekker, maar hij niet (I like eggs, only he doesn't). It also can mean only in the meaning of just, e.g. Zij is het maar (It's just her/It's only her), Het kind eet maar poedersuiker = The child eats only powdered sugar (meaning she doesn't eat 'proper' sugar)
Het has the meaning of it here. You could also change it to: Het is haar maar, but that switches object and subject (so zij turns into haar). There are not many options to change the word order, because when you start with maar this has the meaning of but, also if you change it to: het is maar haar normally (always?) means it is only hair.
I don't think the end goal is to teach your powdered sugar, so much as teach you powdered, and how it can be used. From this (and I could be wrong) I would assume there is:
(if any of these are wrong feel free to let me know :)