Maybe you should take your MacBook into a Kindergarten classroom and leave it with one of the kids for a few hours.
For some people, a laptop is more then just a luxury, it's a tool to get work done, and its an investment in keeping organized, and saving time. Sure it can play movies and music and surf the net, but losing a laptop or having to get something fixed because some parents want to plop their kids in front of it for 2 hours so they can do something else themselves is just not a good idea. If the parents wanted to entertain their kids with movies, they would have brought their own laptop or dvd player.
It's not putting a notebook before family and friends (the people mentioned in the story were not family anyway, but friends, and didn't seem like close friends either). It's an unnecessary risk letting kids use such an expensive device so they can play games and watch movies, when they could be entertained with some cheap plastic toys. If the OP let the kids use it, it's a hassle for him as he has to babysit his machine and the kids. If the parents felt he was watching over it to carefully, they might have felt he was being overprotective about it, even though he was letting the kids use it.
I personally don't like letting people use my stuff. I have gotten back to many broken devices and given the "Oh, no, I got it that way" deal. Broken digital cameras, Playstation 2 controllers, Guitar Hero guitars, DVD players have all come back to me with just a shrug of the shoulders. It is a completely different story when you know someone is responsible, but kids 6 and 3 are NOT responsible. Like I said in a previous post, call it innocence if you must, but expecting them to be responsible is unfair. Value and expense doesn't mean anything to them.
The easiest thing is to just say no and find another way to entertain the kids. Why should some couple guilt someone else into surrendering a fragile and expensive computer to some children so they can watch Donald Duck? I think the parents were being unfair. If the person says no, say OK and move on. Making a big deal out of it shows the couple dont respect other people property, so why would their 6 year old respect other peoples property?