To be planned it would have to be "planned" by someone that did knew that they were visiting and even so it would be very hard to target little Inga when She was supervised almost all time and even when She did went missing the older daughter of the host should be taking a look over Her. Also nothing happen on WH again like this and also if you look at the Initiative Vermisste Kinder - Initiative Vermisste Kinder - Initiative-Vermisste-Kinder - you will see that currently on Germany there are no other "strange" disapearance with the same profile of little Inga on the same region. This excludes a serial killer like MN (of course that one was already on jail when She went missing). She did carry the water but she did ask permission to grab wood for the fire at the edge of the forest so it wouldn't be hard to imagine that She did place the water bottles on the ground and started to look for wood on the edge of the forest wandering there alone. She was already helping so She wouldn't want to go away to other place and miss the fire and the food.