Leave Topple Village via the north-western exit. If you haven't spoken to him already Niccolo will stop you and sell you the Popoi's Notebook for 10 lucre. The path to Cascade Cottage is pretty linear so there's no chance of you getting lost.
On entering the cottage you'll meet with the boy you met in the meadow. After a long conversation the boy will learn how to use magic and asks to tag along with you to Wendel. Since the cave entrance to Wendel is still sealed you'll need to visit Vinquette Hall first to convince the count to unseal the entrance.
Path to Cave
The route to the Batmo Cave is pretty straightforward, as you try to leave the boy will teach you how to jump. Go back to the previous area, then go south-east, go up the first ramp and jump up the ledge to access the new area. Open the chest for a Gumdrop, then follow the new area round to Batmo Cave and a silver Mana Goddess save point.