Most of the time it will be the latter. It is more fun to write and definitely more fun to read. Don't expect professional grabber sentences from elementary or even junior high students, but do urge them to stretch beyond the boring. Suggest opening with dialogue, a quotation, a description of an incident, or a startling remark rather than something dull. It is also great for style development in general.
Good writers have a bag of tricks that get their point across without boring their audience.
Children are unlikely to come up with the narrower, more interesting topics on their own. We need to spend time talking through the topics until we hit on one specific angle that sparks enthusiasm. It provides a clearly identifiable goal or focus. The writer must ask himself what it is he wants to convey to the reader. The reader needs to be left with a challenge, a motivation, or information that he can put to use.