Well, there seems to be a simple answer.
Can they turn anything into a weapon? That one second they are there and when you turn around they're not? You say you often lose track of your child? Or studying the secret art of Ninjutsu. Child Black Ninja Master Costume Do you find excessive dark pants, shirts, and masks? Hm.Sure, normal kids can be sneaky too, and some even disappear on occasion. That they move with surprising stealth, sneaking up on you when you don't expect it? What about their wardrobe? Do they have viper fast reflexes? But yours sound far too good at it.