Children may create their own words based on their understanding of how prefixes (or suffixes or plural endings) work. To illustrate, a 5-year-old girl was being chided by her mother because a doting aunt had given her another doll during a recent visit to the aunt's home. When her mother asked her why she accepted another doll that she really did not need, the girl replied, "Mommy, Aunty insisted, and I couldn't un-sisl!"
It's good to understand this is a normal occurrence at that age rather than assuming there's a learning disability.