There is no better way to ruin someone's holiday than to arm their children with the tools to endlessly annoy them. Here are some suggestions.

After you have children, you quickly learn to fear birthdays and holidays. More dreadful than even the sugar psychoses and attention tantrums is the destabilizing influence of gifts: Into your carefully calibrated world of punishment and reward are thrust these wrapped mysteries from clueless relatives and negligent friends. If you're not careful—if you don't establish and enforce rigid operational security to prevent your children from becoming aware of the existence of a boxed present from Uncle Ed in the closet—you can find yourself thrown into a nightmare of someone else's making.

Here are some good ways to use your friends' children as proxies in a war of annoyance.

Chemistry Set

I don't think these would exist but for gift purchases—I can't fathom any parent voluntarily buying one for their child. The key is buying one for a kid who's old enough to know what chemistry is and that it can cause explosions, but far too young to responsibly or hygienically handle it. The "sweet spot" for this would be about 8 years old. The Thames & Kosmos CHEM C3000, for instance, features all the chemicals and equipment you need to create a "mini fire extinguisher" or "explore the chemical consequences of heat." One of the chemical consequences of heat is your house burning down.
Thames & Kosmos CHEM C1000: $53.90 [Amazon]

"Toy" Ice Cream Maker

This is a twofer: Not only will it induce your target's child to create a shitstorm with sugar and cream, but after the carnage they will have a sugary treat to eat, making a mess all over again and igniting a glycemic rage.
Dairy Queen Blizzard Maker Food Playset: $29.99 [Toys R Us]

Goo

There are any number slime/goo of products aimed at kids, and all of them actually are slimy and/or gooey. They leave a foul-smelling greasy film on anything they've touched, and it's exceedingly unpleasant to find a gob of it underneath the couch encrusted with doghair and food particles, which is what will happen, because the recipient will just rip it into little bits and throw it all around the house with glee.
Gobbledy Goop Half Pound Bucket: $29.99 [Toys R Us]

Anything by Melissa and Doug

Melissa and Doug's line of toys is like the TOMS Shoes of toys—they look hand-crafted and organic and old-fashioned and artisinal and are made by this pleasant self-effacing couple named Melissa and Doug so everyone will buy their toys for your children. They are unfortunately pieces of shit. So if you buy a friend's kid, say, Melissa and Doug's Farm Sound Puzzle, it will seem at first blush that you have purchased them a little toy thing that makes pig, cow, dog, and duck noises. What will quickly become apparent, however, is that it makes but one noise—drowning transistor—and that it makes that noise uncommanded, at all hours, especially when buried at the bottom of an impenetrable pile of toys.
Farm Sound Puzzle: $19.99 [Melissa and Doug]

Veggie Tales

Veggie Tales is an adorable animated series featuring vegetables as heroes. Your targets will be relieved: You mean I can plop my toddler down in front of the television for 22 glorious minutes while he learns to love vegetables? What they may not know is that Veggie Tales is veiled Christian propaganda, designed to inculcate Biblical values. (Which is great! But should be their decision, not yours.) Of course, these things are profoundly addictive, so once the Veggie Tales hook is in, the parents will hear no end of it and will be forced to procure more indoctrinating material for their kid, whether they like it or not. Best for Jewish parents.
Pirates Who Don't Do Anything: A Veggie Tales Movie: $12.99 [Amazon]

Daddle

It's a saddle. For you to wear. While your child rides you around the house like a horse. Again. And again. And again. And again.
Daddle By Cashel: $31.99 [Amazon]

[Top image via Shutterstock]