I think I've said this on here before somewhere, but when I was very young (from about 2ish), I had the phrase 'owy owy cumpa' ('owy' pronounced to rhyme with 'cow-ee'), which according to my mum could be a substitute for a word I didn't know, or an expression on its own. And presumably, anything I wanted it to be.
I make up words now if I can't think of anything suitable that already exists, although they're usually just mutations of 'real' words, so my friends generally know what I mean. I'm not the only one either.
My sweet youngest daughter used to sit in her car seat, thrusting herself backward and forward, sing heartily...3 songs that she made up! Her neologisms were probably more that she had trouble with pronunciations, but they were hilarious anyway. She kept saying, while driving, "Look at all those Hoisonplays." We didn't get it for a long time...then it came to us "license plates!"
My sweet youngest daughter used to sit in her car seat, thrusting herself backward and forward, sing heartily...3 songs that she made up! Her neologisms were probably more that she had trouble with pronunciations.