A great game though a bit more "adult" with the improved script
Final Fantasy IV is a game that has had at least 6 or 7 versions. Starting with the original Final Fantasy II cartridge for the SNES, the game has gone through many translations.
The Nintendo DS game is the most challenging of all of them. You get "augments" that can be added to your characters to improve their abilities or give them new ones. This requires meticulous concentration and giving the correct number of augments (usually 2) to characters when they are about to leave your party. It is possible to miss very important augments early on and thus make the game much more difficult than it would be if you follow an augment guide.
The game is amazing and the story is very deep and more compelling than any Final Fantasy after it, except VI, possibly.
I played this game as a kid on the SNES when it was titled Final Fantasy II here in the US.
Yes there are some topics that are more adult themed, however it is nothing compared to even Disney movies released nowadays. By age 10 kids have a very good understanding of the concept of jealousy and revenge. Those are probably the most adult concepts in the game. The reviews of sex / suggestive clothing? Really? I promise your child has see MUCH less on people at a beach or pool.