Drainage for playground with 12 inches retaining wall?

Question

I am building a playground area, and as part of having the ground level, I ended up building a retaining wall, and I have a question about drainage. Some data on the project:

1. The wall will be retaining 12 inches of soil in a sloped area.
2. The lateral sides of the area, start at 12 inches, and slowly gets reduced until it meets the leveled ground (as you can see in the picture, 12 inches, 10 inches, 8 inches...)
3. I am planning to build the wall with landscape timber.
4. The wall will be acting as part of the border of a playground. It will be a rectangle of landscape timber.
5. The playground area will be filled with cedar wood chips.
6. The playground area is leveled. That's why I did the retaining wall to start with. So, the playground area doesn't have a slope (we are installing a swing playset).
7. I live in Seattle. Not a lot of storms, but constant rain.

I have a couple of questions:

1. Is it recommended to add any type of drainage behind the wall given the height/context of the project? And if so, what type? Gravel, Gravel+Pipe? If I do piping, where should the pipe output go to? At the end of the slope I have my neighbors fence.
2. Should the drainage also be added on the lateral sides of the playground?
3. Would it be enough with just covering the soil with some landscape fabric, put the timbers on top, and then just covering with the cedar wood chips?

Here is a picture showing a part of the area. The other lateral side looks just like the one shown on the picture.

Answer 1

If you want a sharply walled area, presenting a clear divide between the lawn and the cedar chip play area go ahead and make your wall, but give some consideration to what it is holding back, and what is going to keep it in place.

1. No. You could add drainage, but the main purposes of drainage in a retaining wall are to reduce pressure on the wall due to the mass of water pushing on the wall, reduce soil movement (erosion) that could undermine the wall, and soil movement (liquefaction) that could compromise the structure. For a 12" wall the pressure from the mass of water and soil is not large, and with clay soil the water is simply going to flow over your wall rather than under it. If you do add drainage, it comes out under/through your wall, generally to the flattened area but can be anywhere conveniently farther down slope (note if there are city ordinances about directing water onto neighbouring properties - you may have to be a certain distance within the property line).
2. No. You could extend the drains, but it would be even more overkill than point 1. The shorter slopes will have even less pressure.
3. No. Here we get to the real crux of the matter. If you just lay the timbers on the dirt and against the dirt wall, what is going to hold them in place? Nothing, so they will be free to move, tip over, in colder climates they would heave in the winter, etc. so you must give consideration to what this means for your project. In points 1 and 2 we talked about the water pushing on the timbers, but also the dirt itself will be pushing on them, people walking on them, etc. that will tend to push them up (dirt settling below them) and down slope (dirt settling behind them). You will want to look at pinning them into place (basically wood or metal driven deeper so that the timbers cannot move without the entire pin also moving). My suggestion would be an earth cut, which just means you dig a trench deeper than the wall itself so that soil on the lower side holds the timbers in place against the pressure from the upper side (e.g. a 24" tall timber with 24" of dirt for your lawn, and 12" of dirt for the cedar chips). There are many many variations on how to pin the wall in place so that it does not move.

But in the end, it's just a 12" wall for a play area. You don't need to overthink it or overdesign it. A few timbers without drainage and with minimal pinning may start to move over time, but your house isn't falling into a ravine... as the kids get older the playground might no longer be a playground, or you may need to dig back the wall to reposition the timbers and refresh the chips. It's not a landslide.

The simplest option... grass makes for a great erosion control system to stop soil movement. Cut back that slope a bit so that it's mowable, plant some grass or sod, and call it done.