The Spiderwick Chronicles Cheats for Nintendo DS
While crossing the first section of swamp, you will be attacked by a river troll and it's smaller version. Use your confusion fairies on the smaller troll as it will not work on the larger.
In this way, when the big river troll grabs one of the kids, the smaller one will attack the larger and set free the child, leaving you more life and more open hits towards the larger troll.
Kill the larger troll first, then the smaller one. If you do this, you will defeat them both. It may take a few times for you to get the hang of fighting them, but I found this the surest way of killing both of them successfully.