Police said high winds caused a 30m tree to fall on the group about 11am, pinning four people beneath it.
The 21-year-old woman and the 16-year-old girl died at the scene despite the efforts of first responders, while the baby was airlifted to hospital where she died.
A young woman, aged 18, was also airlifted to hospital with serious injuries.
Photos from the event show brightly coloured Easter eggs and bunnies scattered throughout the woodland.
Metro reported that little was known about the condition of the fallen tree.
SterniPark, the organisation that runs the residential group, said the community was “mourning”.
“A falling tree brought the joyful Easter egg hunt to a terrible end,” the service said.
“The residents, children, young people, relatives, and staff now need time and our full support to come to terms with this terrible tragedy.”
The organisation thanked first responders for their support, including crisis counsellors “who did everything humanly possible on site to help, to rescue, and to offer support in this time of grief“.
Schleswig-Holstein Minister-President Daniel Günther said the deaths had left the region “deeply shaken”.
“Our thoughts are with the families of the deceased, the injured, and all those who had to witness this terrible event.
“In these difficult hours, we wish those affected strength, support and the assistance they so urgently need.”
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