Cut a piece of 1-inch-thick, 6-inch-wide pine to a length equal to the width of the slide with a miter saw.
Drill four evenly spaced 1/2-inch-diameter holes across the middle of the board. Space them so they run evenly along the full length of the board.
Paint or stain the board to match the color of the bed. Allow drying time. Position the board against the lip on the top of the slide. This is the part of the slide that typically mounts to the deck of the playhouse. Mark the holes onto the plastic lip, then drill the holes out with the 1/2-inch diameter drill bit.
Place the board against the side rail of the bed frame and line it up where you want to install the slide. To minimize the space between the mattress and the top of the slide, line the top edge of the board up with the top edge of the side rail. Mark the holes onto the bed rail and drill the holes out with the 1/2-inch-diameter drill bit.
Sandwich the plastic lip on the slide between the bed rail and the wooden board. Slide the washers onto the bolts and slide the bolts though the board, slide and bed rail. Place washers onto the ends of the bots and thread the hex-nuts onto the ends of each bolt. Tighten the bolts with your wrench set.