Distraction, via food, voice or otherwise, is usually a good way of managing in the short term but in my personal view it can take quite some time before the dog gets the connection if you're using distraction alone and relying on things to happen (i.e. whenever a child happens to pass by).
Another short term technique to can be of help is finding an incompatible behaviour, i.e. one that, if he performs that behaviour, he can't exhibit the problem behaviour at the same time.
In your case, for example, you might ask your dog to sit and to watch you. Both behaviours are incompatible with lunging, i.e. it can't lunge and sit at the same time. But you'll have to ensure his motivation to keep sitting is higher than his fear of the child. So get not just your average treats but very, very special super yummy sausages, steak, whatever he likes most. This is different from simple distraction, as you are asking your dog to actively do stuff and he knows that lunging at children will mean losing out on the reward he knows he's getting if he just keeps still that little bit longer.
The long term solution is what's called desensitization and counter-conditioning. In short, it means gradually getting your dog used to whatever triggers the fear reaction (and it is fear 99% of the time rather than aggression) while at the same time associating the trigger with positive outcomes.
Practically, it means you'll want to recruit someone's child to help you, since children seem to be the trigger. The main thing to do is to make this training exercise safe for the child, your dog and yourself, so ideally set it up with a fence between the dog and the child, so the dog can't get at the child even if you lose control of the leash.
Find the distance at which your dog is still comfortable with the child, i.e. well before it starts lunging or showing any other warning signals (growling, raised heckles, etc.). While you are at a comfortable distance, whenever the dog looks over to the child, reward it with lots of praise and food (or a toy, if that's what motivates your dog). Really hammer it home that good things happen when it looks at the child and doesn't react. The best way to do this (if your dog is food motivated) is to give him a special type of treat, the nicest you can find, that he only gets when he is around children and behaves himself. Don't use this type of treat for anything else!
Once your dog is happy with the child at a certain distance and realises good things happen with the child around, move a step closer. Really go slowly, as getting it wrong will undo a lot of work up to that point.
When you do get it wrong and your dog reacts to the child, lead it away to a safe distance and start again. Don't punish the dog! You're the one who pushed it too quickly into a situation the dog is not comfortable with. Simply go back to a distance the dog is ok with and start again.
Do this for 5-10 minutes, then wait until the next day. Gradually build up your dog's resistance to children and slowly associate them as being good for it.
Keep in mind that this might be a long and slow process for the simple reason that when you're actually out on a walk and your dog is forced into a situation it's not comfortable with (children have the unfortunate tendency of wanting to pet even the most fearsome and aggressive looking dogs) and it lunges, the kid will probably run away, meaning the dog's response just got reinforced and you have to undo that again.
Dog is scared of kid, dog lunges, kid runs away => dog achieves desired outcome.