[Closed] Building a large wooden swing frame for the kids - how to fix it into the ground

I did a single swing for the small boy a few years back and I would think about what happens in five years when the children aren't so bothered with a swing. By this time the chances are that timber will have started to rot at ground level

First thing to consider is whether you want to put the poles directly into the ground or whether you can use a metpost-type arrangement to avoid rot and concrete. An annual poke around at ground level with a screwdriver will help monitor rot, then an inspection of ropes, joints, etc

Less concrete used means less to remove when you want to reinstate the lawn; I used a 6" post-hole auger and worked by eye off an erected frame to get the angle. You might be confident enough to use a 4" auger, I wasn't  Don't forget to space the holes according to what will be ground level on the frame. If the length of the base of the triangle is adequate, then 2' will be adequate in the ground; scale some photos or measure some examples

I can't help you massively on crossbar dimensions, it'll vary according to child size and swingage

We were given a swing kit which was similar and it came with metal straps which you bolted to the legs, and concreted into the ground. Doesn't move even with an adult on it!

it came with metal straps which you bolted to the legs, and concreted into the ground

Yep that is what ours uses. I set the metal anchors in tubs of concrete about 1' deep and 6" cross section then dug a hole and set these in the ground with the legs resting on them. Putting the concrete in tubs meant I could move it around to get the feel level and can remove it easily if I want to move the swing to another part of the garden.