Here's another example of a great site I think most children will love. It's done by a paper aeroplane enthusiast: Alex's Paper Airplanes. It has video and print directions, plus diagrams that teach you the necessary folds and cuts. There are some fairly unobtrusive Google ads, but the written directions are clear, and there's even a flying lesson to get you started. I went flying yesterday with the little helicopter! Fun Paper Airplanes is a different site with free downloadable templates for planes, which might be useful if your kids are younger.
If paper folding in general interests your children, try Wikipedia's guide to origami folds and Tammy Yee's Origami Page, or check out my article Fold Me Up, Scotty, particularly the comments where author Sandy Fussell gives a neat link on Origami and Maths.
Constructing and flying paper aeroplanes is a wonderful activity for the holidays or a wet weekend. Institute a challenge like who can invent the plane that flies highest or furthest, or land on a precise spot, or loop the loop. Offer prizes or a trophy too if you want. Kids are investigating principles of flight, and developing skills in creative and analytical thinking, hypothesizing and testing theories. It involves them in a scientific activity, but best of all it's fun! If your children want to follow up and explore more about flight, perhaps learn about gyroscopic procession and torque, try NASA's Aerospace Activities and Lessons.
If your child is particularly interested in an activity, maybe they could write clear directions for it, or create a how-to video about it. You'll find lots of video examples at Youtube.
(Cartoon created at ToonDoo)