It's essential that a child's swing set rest on level ground, not only so the swings move correctly, but also for the safety of those on it. If the set's frame is not level, it will only take one rambunctious swinger to bring down the entire assembly. This project is not very difficult, but it is labor-intensive and you may want to enlist a friend or two to help. It will most likely take an hour or two, possibly all day depending on the size of the swing set and the condition of the ground.
Measure out the area for the swing set. The directions that came with the swing set should give you the dimensions that you need. Place stakes at the corners and run a string line between them so you have a straight line.
Start at the lowest spot in your area and dig down 2 inches. Then move on to a deeper section and dig down until it meets the first section. You can use a flat head shovel after the first section and scrape the rest. It will make it easier to keep it level with the first section.
Check the level both ways by laying a 2-x-4 board down on its side and placing a carpenter's level on top. Make sure this section is level before moving on to another. Always dig down to level and never add to the ground to make it level. The added ground will be loose and can cause the swing set to shift.
When the entire area is level, assemble the swing set. If it has posts, you may want to insert patio pavers for each post to sit on. Make sure to level the pavers, too. After the swing set is installed, put down some landscape fabric and fill the area with 1 1/2 inches of mulch. This will keep the play area from getting too muddy and will make for a softer landing for the little ones who fall.