Greater Manchester Police said five people, aged between 20 and 70, had been arrested and bailed on suspicion of concealing a death and unlawful burial.
The remains were discovered overnight on Friday into Saturday in the Marsh Green area of Wigan.
It is understood that those arrested have special needs and are all known to the child.
It is thought police responded after being contacted by social services.
Chief Superintendent Clare Jenkins from our Wigan district said: “This is a truly heart-breaking discovery, and I do not underestimate the impact that this news will have on the local community.
“I would like to reassure you that we have several teams of officers and specialist resources working diligently to find answers.
“You will likely notice an increased presence of our officers throughout the next week, if you have any concerns or want to share any confidential information, please do not hesitate to speak to them.
“In the coming days we will know more about the circumstances surrounding this incident, but we are confident that this is isolated, and there is no threat to the wider public.
“At this stage we are keeping an open mind, and we will provide updates as soon as we can.”
No-one else is being sought in connection with the death.
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