Try a water fountain.

Active noise canceling headphones work because they can physically separate the input and output sides. Outdoors there are just too many surfaces for sound waves to bounce around.

Your best bet is to plant trees and bushes around your fence line because other sound absorbing materials (like those blankets used on construction sites) are going to be pretty ugly.

THC works

Suggest bushes, water fountain or a noise-generating water source Probably a losing battle but perhaps something like cork or wood lattice-type structure to absorb sound - that is commonly used in noisy places? Try talking with you neighbor to see if the children's play area can be slightly relocated. It's not worth a lot of conflict. Having kids play and hearing the fun can be nice too. If the sound it inside your house, there are more expensive things like double wall windows, check out house in area in airline flight paths. They have several noise reduction issues that have applied. The teenagers next door played basketball late into the night. We were glad to see the kids having fun and let them know it. It subsided after a few yrs. No conflict. Your issue is very different from my first house where having a stupid neighbor that plays loud music and blows up firecrackers in your front yard and drives his truck on your yard and knocks your fence boards out so his dog has a little more space to run. For that problem, you may want to move.