In reply to Richt79:
My gut feeling is most seven year olds are only going to train if it's something they enjoy. I've never met anyone who enjoy lunges, so I would expect it would be fun to show him lunges, he may do them a few times, but quickly get bored of them so it won't really translate to any benefit.
Naturally, I don't know your kid so I'm basing this on my interpretation of a typical seven year old.
Running faster is about strength and fast twitch muscles, so intervals both flat and hills would be the way to go.
Kids are great of not over doing things so he'll hopefully naturally limit his effort to what he wants to do.
Now I may as well go and contradict myself, but school really put me off running. We were taught to run as hard as possible to get fit. I was naturally a slow burner so never enjoyed running at school and the teaching methods meant I didn't run until late 30's. now I blame the teaching methods, but it was probably me. What I'm trying to say is it needs to be fun.
If he enjoys football that's a great interval training session so maybe he's already doing what he needs to do to get faster.
Sorry I've probably just confused you haven't I?