Don't worry about the interesting opening. It can be added later, when you're typing it into the computer. Right now, you're just getting the facts on paper in a meaningful way. Once it's done, get it typed up. If your child isn't typing at all yet, get him started.
Children can edit much more easily on a computer.
If he can type, even if it's only hunt and peck style, let him type parts of the report. Help him out a little until he's a little older, but don't change anything when you type. You'll be editing together later. Once it's all in, decide on an introduction. Try reading some introductions in his books to help him get ideas.