The small-print on the spy pens packaging said that the invisible ink is harmless to the skin and tasteless. Aren’t those the first questions everyone who works in espionage asks – will this make me break out in a rash, and does it come in mint or vanilla flavour?

OK, probably not, but those are important questions for a family bash where someone brought two dozen invisible ink spy pens and every kid in the house is in possession of at least one.

The writing is invisible until a UV light on the opposite tip of the pen is flashed over it. Someone made the mistake of announcing that the packaging said the pens can write on T-shirts, paper, skin and any surface. The ink, it said, is "permanent, but washable."

On hearing that, a five-year-old lunged for the sofa with her invisible ink pen drawn. She was intercepted and kept under close surveillance.

One of the kids who received a box of "You’re the Best" notecards, took his invisible ink pen and wrote his name at the top of the cards. He was now the best. The people he wrote the cards to would not know it, but he would. He beamed.

Caps flew from pens, UV lights flashed around the room, and soon little ones graduated from writing on paper, to writing on their own arms and each other. The kids weren’t the only ones.

A toddler that disdains clothes and yanks them off about four times a day was walking around shirtless. She stumbled into the path of my UV light. Her chest said, "I love my Aunt."

She’s 18 months old. She didn’t write that herself.

Not to be outdone, the toddler’s uncle drew intricate wings on her shoulder blades, revealed with a decoder light held by this concerned grandma.

"What’s the matter with you people writing on the kids?" I scowled.

Just because there are cool invisible ink pens doesn’t mean the adults have to act like children, too.

Later that night when the kids had on pyjamas and were ready for bed, I sniffed a few heads to check for the fragrance of shampoo. Detecting none, I grabbed an invisible ink pen and wrote the date on the back of their hands. If I can read the date with my decoder light the next time they spend the night, I’ll know they’re not using soap.

"It won’t be pretty," I warned. "You’ll all need to go undercover."

It’s quiet today. They’ve all gone home and taken their invisible ink pens with them, except for one I kept for myself.

I’ve considered scanning the walls and the furniture, but I haven’t. What I don’t know won’t hurt me (said no spy ever).

Today I received a text saying someone left a note for me at the bottom of my desk calendar.

It looked blank. I retrieved my spy pen and turned on the UV light.

I think you’re wonderful, too, Sweetie.

More from Lori Borgman: