The monster in the pond

I was outside with my dad and possibly my brother and pet. We were supposed to go fishing for dinner. Our massive pet dove into the water. It either swallowed or was swallowed by some gigantic sea monster whose maw stretched the breadth of the body of water. The pet had tried to save us, but was unsuccessful. We ran into our barn-house and the monster followed. Somehow, I knew that magnesium would hurt it, so I tossed a magnesium supplement pill through the cracks of where I was hiding. While the pill bounced around and the monster skirted around looking for it, I made my move and collected my bottle of pills and made my way up the ladder in the house to a secret door in the roof by which I could escape. Through here, I left, but made my way back inside through another panel in the roof and dispatched all the magnesium pills to the monster. That's all I remember.