Yes, if they can play the Lego games well I think they will be fine with this game. There is a lot of text in this game they will have to read, but as far as the puzzles and things go, I think they will be fine. This is probably a little harder than the Lego games, but I think if they can handle the Lego games they will do well with this.
yes, 3DS is the next system after DS. DS games can be played on a 3DS, but it doesn't work the other way around. The system has gone down in price since its release, and you might be about to find a special offer for it due to the holiday season.
Are you people out of your minds? A 5-7 year old would NOT be able to handle this game. It has lots of puzzle solving, platforming, and confusing boss fights. Reviews have stated how overly difficult the boss fights are. This is not appropriate for anyone under 13 at least. You need to be able to read and follow a guide for this game.
I don't think it's a good idea to buy this game for a small child because you come to areas that you aren't able to access and will have to go back to the scene at a later date (the young may not tolerate this), getting clues from Kersti if you're stuck and managing inventory.