You either have to raise or lower the uneven part. If it's a lot and you can afford it, hire a landscaper or someone with the heavy equipment to do it. If it's dirt, you will not want to leave it just bare dirt for the swingset area, so either wait for the lawn to grow or plan on what you want to fill it with on the surface. Gravel is common, affordable, and painful when you fall. You can get mulch. Wood is cheaper, but is a mess for a while with the bad smells and moisture. You can also get shredded rubber, which does last a lot longer, but also smells for a while.
Edit... A simple way to raise it, go to the highest point and hammer in a stake. Tie a string to it and stretch across where you want to raise and tie it off on another stake. Get a bubble level and hold it up to the string and move the string up or down the stake until level.