I like the idea but I don't have high hopes for it as I suspect it overlaps too much with an existing SCP (at the end). Might as well try though. Please also tell me if it's rubbish or too silly/unbelievable. Helps me to get my compass aligned to everyone else's.
I have two possible variants. I know it's missing something in the features department, ie "what else does it do?", I'll think about that later if this initial idea is okay. This isn't meant to be proper formal tone text below, just a rough description of the idea, thanks.
Variant 1) A full body man-shaped fabric costume with a zip on the back. Very simple design. It walks about. Presumably there is someone inside the costume but we can't see who because the costume covers the entire body. The occupant speaks in a child's voice and exhibits a child's maturity, expressing only very simple emotion based ideas, etc. Despite having a child's voice the occupant and costume is about 6 feet tall.
It acts happy, curious, and when stressed, scared. Much like a docile animal. It is never aggressive. It is very paranoid/defensive about the zip running down it's back. It won't willingly let anyone behind it. When forced against it's will and unzipped nothing is found inside the costume and the costume goes limp. Just an empty costume. Zipping it back up has no effect. If the costume is in this empty state it will just hang in a crumpled heap like a regular empty costume indefinitely.
If someone, eg a test subject, gets in the empty costume and zips it all the way up, things are normal for a while. They might joke as they walk about and mimic the child entity. Within about a minute the occupant falls silent, arms drop by it's side. Attempts to get the occupant to respond fail, until suddenly the child's voice speaks out. The "child" is then back and no trace of the human occupant is heard.
Force it down, unzip it and the suit is found to be empty again. The test subject who got into the costume is nowhere to be found. At this point someone else can get in the costume and zip it up and the "child" will return. This can be repeated again and again.
Variant 2) Instead of being empty the suit is found to contain a dead body when unzipped. zip it back up with the dead body in and the child returns. Remove the dead body and zip it up, nothing happens. If a living person gets in the empty costume and zips up, the child returns. Unzip the costume and you find a dead body. The child asks questions about things only the dead person inside them could know, implying that they have access to their knowledge. So it's similar, but a bit more morbid.
I actually prefer variant 2 because it has more scope with the whole "child has the dead bodies memory" but it's more morbid and I don't think a lot of people will think I've done it to be morbid.
Biggest issue though is this: http://www.scp-wiki.net/scp-1545. How close is this? Is the child different enough? Costume check, people go inside costume check. Is this going to be another case of "it's already been done!" defeating me? (although that would be better than the "that's a shit idea!" which usually defeats me).