My first question is how old is the child? Most kids outgrow play space quickly and sand boxes invite the neighborhood cats to use it as a litter box. I like terracing of slopes but what you show and describe is hard to understand. Outdoor spaces IMO need to be kept pretty simple with good design. I for sure would not mix real and fake grass IMO you need a good landscpae designer/ contractor I think you need to remove all the stones and bricks you have outling areas for sure , too much going omn to even begin to understand the space . I see one area in the background with gravel and some brick next to it so alrady mixed finishing that IMO cannot handle more and differnt finishing . But trully I am confused as to what you really want.

My experience is that children often like to play near the adults, so I'd focus on making the area inviting to all ages by providing seating, a place to set down a beverage etc.

Leave grass space for larger toy items such as an portable sandbox with a lid (e.g. the turtle ones), a small splash pool etc. Maybe a small metal-frame fabric gazebo to protect everyone from the sun on a hot day?

For example, my grandchildren recently spent most of the day mixing "potions" in my big metal mixing bowl, using dried out play dough and leaves, pine cones etc. Many of my kitchen tools were involved e.g. plastic measuring cups, soup ladle, strainer, funnel. But they wanted to play on the outdoor coffee table right where we were sitting, not off in their own area.

We did about the same area for the grandchildren. We bought the Step Two stuff. A brown sandbox and a picnic table. We also bought a low budget swing with a small slide.

The sand box had a top that fit well and was easy to remove.

All of it we sold easily online after they outgrew it. Only the swing had to be anchored in the ground and it was easy to remove.

I did not use fake grass, we used wood chips, but the fake grass really would of been better. All was easy to remove when not needed.