Is it a suitable place for a planting bed?

It might be easier to find perennials and shrubs that can take the sun, instead of grass. Then you wouldn't have to mow the slope, and could hide the manhole cover. Shredded bark as a mulch should keep the soil from washing until things get established, as long as it's not too steep.

Treelover's suggestion is a good one. Also wonder if building a frame for a ten by ten sand pile be too much? I know my kids spent hours and hours in their sand pile and would have been in 'hog heaven' with an even larger one. Perhaps a small wooden jungle jim could be built within it -- throw a tarp over it and make a play tent. Kids love that!

Just an idea not knowing what the area is like, how much of a slope there is, and whether it is in a suitable spot for your children to play.

Be careful about using mulch with nothing to hold it in place. With the torrential rains we have in the spring mine just washes off my slope and into the adjacent grass.

I totally would do the sand thing except we have a ton of feral cats. Not a healthy environment for kids to roll around in, once the cats go digging in it. Otherwise it would've been a great suggestion.

The area is near a far corner of my yard, so it might do better as a planted area; maybe I could even do a retaining wall to level it? Hrm ... I ought to get a picture and stick it up.

BWAHA, I have a solution! I took down a raised bed in front and used the ties and the dirt from that to make a big bed over that problem area! I reinforced the back so the rain won't wash the soil out, amended with some humus, going to add some more compost, and I'm going to see if I can get some vegetables to grow. :) The manhole I won't be able to grow on, so I'm going to put a big planter on top of it and fill in the area around it with mulch! Tahdah!