Travel on whale
In order to get to the Lavalava island, you need to help the whale. A fuzzy is in its stomach and you need to catch it. Use Watt to do so – first make him run around by using Watt's special powers. Once he's jumping around, watch where he lands. Go to the place that you can reach and once again use Watt's special powers. When the fuzzy jumps on you, it'll decide to fight you. Beat it and you'll be able to travel on the whale.
Hide and seek
You find the first Yoshi Kid in a bush. The red one is found first. But before that you should get Sushie on your party. When you are in Yoshi Village where everyone's going crazy, take the western exit. From there, there will be a full row of bushes blocking your entrance. Go to the center of that row and push 
. Swim west from there, and on the westernmost half of the next screen, go to the dock. Again, bushes will be in your way. The first one is just a gate and the second one is the key to getting the red.