I have read some reviews that these pull out too easily. They must be hammered in at a 60 degree angle leaning away from the tent. No, they won't hold down a bounce house. They're just tent stakes. Yes, the plastic will break if you hit it with a hammer. You can move the plastic down until the end. The manufacturer recommends leaving 1 and 5/8" out of the ground, so you can move the plastic up at the end. Also, just use a regular claw hammer- you don't need a sledge hammer on smooth stakes.
I am happy with these stakes. You get 10 plus an extra plastic topper. I have two tents for earthquake preparedness as well as camping. I am going to order another set. I don't know what I am going to do with the thin rope (thick string?), but it may be good to help tie down the fly on my oldest tent.