Can I convince you to not do this ?
Honestly not trying to be an ass.
Please, please - Get a quote from a company that builds retaining walls.
The quote will cost very little or nothing.
You will be shocked at the cost, and not in a good way.
The quantity of material needed to build the wall, will amaze you.
Please get a quote or two first.
I promise you, you will not regret it.
Block walls work in compression, heavy things directly on top of them is fine.
Side to side, their goal in life is to tip over, and they are damn good at it.
Blocks work because they are tied to very solid, heavy things at the top and bottom that run the length of the wall, and have regular supports at a right angle to the length of the brick.
A 1' deep pile of soil, behind a 4' block wall, 1 block long, weighs 500 pounds.
The path of least resistance is to knock over the wall.
Blocks, attached with mortar, to the soil behind the wall about as strong as a 2x10 resting on its edge.
The question is more one of how many yards of concrete will you need to build the footing.
If properly reinforced and if the actual footing (which the 12" column isn't, unless you have no flare or flat pad on the bottom, which you should reconsider unless you have stunning bearing) is of adequate size to support both the imposed load and the weight of the longer column, it should not be a problem.
But it's difficult to accurately assess such things from afar, or to be certain that it will be safe in your particular soils and configuration. A few hours of a civil engineer's time to establish exactly what would be safe is probably money well spent on peace of mind.
This article might be inormative even if it's not all that DIY oriented. The sidebar suggests that the minimum cover (concrete between surface and steel) should be 3" on the bottom (earth) side of the footing, and 1-1/2 inches on the sides of the column, assuming you are using #4 bar which is pretty typical for small projects. So you might use 6 #4 bars in a 8-9" circle, bent outward on the bottom and tied together every 8-12" vertically. If you really want to be the oddball overkill homeowner you'll figure out a way to bend a bar into a spiral to tie the vertical bars together, just like the big boys do. The rest of us normally compromise with wire, other than the ones that hope fibered concrete is all the reinforcing they need.
Best Answer
If this is a kids sandbox I'm assuming they'll likely outgrow it at some point. When the time comes to remove it you won't want rebar in there. If it doesn't serve any other purpose than a border for the sandbox then I'd skip the rebar.