Get the kids away from the electronics and out in the fresh air by installing a tire swing in your backyard. Tire swings have been around forever because they’re fun for any age and they're inexpensive to make.
You can use the same knot to secure the rope to the tire and tie around the tree.
There are as many opinions about which knot to use as there are knots, but there are two general types you can use: a loop or a hitch.
The most simple loop knot is the bowline and probably what you picture in your head when you think of tying a knot.
Toss the rope over the limb, leaving about a foot extending past the limb on one end. Now make a loop on the longest side of the rope just below the limb.
Pass the other end of the rope through the bottom of the loop, then around the long end of the rope, then back through the loop.
Pull to tighten.