From kids in our family, words we have kept:

Clo - the singular of clothes; ie, garment.
Disea - the singular of disease; ie, illness.
Rup - Europe; once when I told Wil that Mom was in Europe, he thought about it and then asked, is that my rup AND Timmy's rup?
Uck - stuck, as in, I got uck behind the couch.

and there are others I am not thinking of.