Children will be children. You just have to laugh.
When my grandchild was 4 years old, I explained about fibbing.
I said, "Do you know what a fib is?"
The grandchild said "No"
I said "A fib is when you are not telling the truth. Do you understand now?"
"Yes", said the grandchild.
"Well" I said. "When a grandchild tells a fib, a little red spot shows up on your forehead that only a grandmother can see."
"Now, I will ask you again. Did you make that mess in the loungeroom?"
The grandchild straight away placed its hand on its forehead and said "It wasn't me".