Most consumer electronics don’t come with the cables, because cables are a high-margin, high-markup item. Many people don’t know that. And many people give and receive consumer electronics on Christmas day.
If you want to be a hero, I recommend paying Monoprice a visit. Monoprice sells all of the most common cables at deep discounts. I recommend picking up a couple of the most common types of USB cables, an Ethernet cable or two, and at least one HDMI cable to start with. If you think of others, go for it. For around $20, you can build up a sizable emergency kit and probably still spend less than the cost of just one cable at a big-box store.
And if this is the first you’ve heard of Monoprice, pay them a visit. They’re the best source I’ve found for inexpensive cables of reasonable quality, and if you want premium cables, their premium offerings are still reasonably priced.
One more thing
If at all possible, I also recommend testing electronics ahead of time. My uncles made a Christmas tradition of testing anything and everything electric before anyone wrapped any gifts. If it plugged into the wall or used a battery, they tested it behind closed doors. If it made noise, so much the better. They had a lot of fun with it, and making us wonder what on earth was going on back there was a big part of the fun.
If something didn’t work, they did this early enough that there was still time to exchange anything broken. For them, making the rest of us wonder what they were doing was enough fun to make it worth it. But they were doing everyone else a favor too. Nobody ever received a gift that didn’t work.