95% of the time spirits that appear as children are not children. It's a way that shapeshifters (generally the trickster variety) assume an innocent or non-threatening appearance. Most people act very lovingly toward children naturally, and this ensures that they will receive not only your attention, but your passion. Be very careful. Some will tell you that they are actual children and act like they need your help in order for you to do something for them that gives them more strength to influence the physical.
Children rarely if ever "haunt" this plane. They move on quickly.
I have once encountered one childlike spirit that did not have malicious intentions, but genuinely was very timid and "youthful". He was an old soul and a nature spirit. Some house spirits are also known to appear as children due to their short stature, although there are key features that tell them apart, generally ones that demonstrate that they are not quite human.