Need ideas to keep a sheet on a mezzanine bed. Size Twin, a child restless sleep, nothing works!?

The three of my teen have loft beds, but with a size of rooms - we can not keep a cloth on it for more than a day or two. This is a good-sized mattress (not is a pillow, but not a small, cheap one either). It's a very restless sleep, making it difficult. BTW, this is a normal size teens, 5'8 ", 150 lbs. There are problems with the other two, but have softer mattress Ikea, which can make the difference. IDEAS? Yes, the fitted sheet, do not use the upper leaves (only comforters). Important Safety Pin pulled the sheet (I used six around the bed). Love founded the "road idea, and clips. I'm so frustrated by it (silly, I know) I'm not just sitting back and enjoying the challenge!

I guess U: the fitted sheet sew the straps that go under the mattresses in all of fixed width with Velcro or you could make a blade that falls like a pillowcase

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