This is really my elderly friend's recollection.
Her family bred Siamese cats, which were of course good mousers. When the little girl learned to sing, "Gentle Jesus, meek and mild" she thought the 3rd line went, "Pity mice implicitly" instead of the actual "Pity my simplicity"! (You must remember that children's spoken vocabulary was much more sophisticated in those days!)
A childhood belief about hymns

others believed things like this
- I used to think the song"jesus, oh what a wonderful child so holy, meek and mild" was "so holy, Mickey Mouse"— Stephani
- I thought when someone was described as being "mild-mannered" it meant they didn't have very good manners— Samilton Duerheiser
- I used to believe cats ate dogs and mice ate elephants— lucien
- My mother, the good Catholic, thought that Jesus, being a string bean (supreme being) was really skinny.— Mark D
- I used to believe all dogs were male and all cats were female. I still call dogs "boy" and cats "girl." — Kelly F.
- My parents told me that Jesus visited Santa Claus when he was dead for three days. — Rick Hamell