This is just my opinion, but it seems to me that these scenarios are too nice. It doesn't feel like there's enough of a threat to compel the players.
The shapeshifters in your first scenario aren't really doing anything evil. At the end of the day, it's a little kid who happens to have powers who is shoplifting some camping gear. I think these would be a great red herring for something more sinister happening behind the scenes. Maybe if you have it that a powerful object like an idol is stolen inadvertently by the kid shapeshifter from cultists who were planning on using the stolen object in a forest ritual? The cultists pass themselves off as normal, yet aware mortals, who persuade the players to help them retrieve the object from the "evil" shapeshifters (that will probably use it for the evil ritual that the cultists were going to us it for in the first place; the sacrifice of the shapeshifters will add power to the ritual. The players find out the truth and find themselves having to rescue the shapeshifters. At the end, perhaps the shapeshifters make the PCs honorary members of their tribe if they are rescued.
For the waist deep scenario, don't send the creature back to the NeverNever if the players don't do anything. Make sending it back the goal of the encounter. Also, the St Louis Arch is pretty big, so you can summon something the size of Godzilla if you want (give him Predator style cloaking/invisibility as well so that the destruction gets explained as an earthquake). Godzilla tries to hide and leace the city, but he leaves a trail of destruction in his wake. The players know that something is wrong, and are told they have to stop Invisible Godzilla, so they set about tracking him down outside the city. They go to a museum and they come across an artifact that allows them to See the Invisible (or have access to an enchanted object or potions or the like). The summoner's goal was to control Godzilla, but things didn't go as planned, so he tries again, and ends up summoning something that is Mothra-sized comes through. This time he gains control, and sends Mothra after Godzilla to control Godzilla as well. Godzilla and Mothra meet. Let them fight... Let them fight! The players have to contain the damage, find the artifacts that let them see the Invisible monsters, find the summoner, take scroll that controls Mothra and opened the gate in hopes of banishing the creatures and closing it. Along the way, one or two allies from the NeverNever might get involved, but if they try and take the lead in banishing the creatures, they are killed or incapacitated. However, they provide tools and equipment that the players can use. Eventually, the players realize that Godzilla just wants to be left alone and return to the NeverNever. They have to find way to banish Mothra, and return Godzilla safely.
