From the Super Mario Wiki
World 5 is the final world in Yoshi's Island DS, before Bowser's Castle. As with all worlds in the game, it contains eight normal levels and two secret levels, one unlocked for beating the game and another for getting 800 points in the world:
The boss at the end of world 5-4 is Moltz the Very Goonie. In the final level, world 5-8, Baby Wario and Baby Bowser rejoin the group as they head into Bowser's Castle. However, at the end, Baby Bowser betrays Yoshi and fights the player, but is defeated. After that, Bowser, in his normal form, shows up and attacks as well. After he is defeated, Kamek uses his magic to enlarge him, much like he did to Baby Bowser in Super Mario World 2: Yoshi's Island. In this final battle, the enormous Bowser is in the distance and the player controls four Yoshis at once, carrying Baby Mario, Baby Peach, Baby Donkey Kong, and Baby Wario and must throw giant eggs at Bowser to defeat him. This is also much as the final boss of the original Yoshi's Island.