"The children run through the restaurant."
Translation:Barnen springer genom restaurangen.
I am not entirely certain of the differences between the two, but I read elsewhere that "genom" primarily plays the role as a preposition. "Igenom" can be used as a preposition as well, however it can also play the role of an adverb.
English examples of "through" used as a preposition and then as an adverb:
"I see through you." [Preposition]
"Your document submission went through." (i.e. Your document was successfully processed, etc.) [Adverb]
As I mentioned above, I am not completely versed in the appropriate uses between the two words, but this might be of some use to you and others -- at least until a native/fluent speaker can correct and/or enhance my response.