Measure and make note of the dimensions of the child's bed. Visualize how you want the tent to look, especially its height, so that you can purchase an appropriate amount of fabric.
Choose a fabric that appeals to your child, and get enough of it to make the tent as large as you would like based on the measurements you took. The length should be about 6 inches longer than the length of the bed, and the width should be enough to arch over the bed at your preferred height.
Fold over a few inches of fabric along the two sides that will arch over the width of the bed. These will become tubes through which the pipes will slide, so be sure to fold over enough material for the pipes to fit. Safety pin the folded edges in place.
Sew along the edges of the folds, removing the safety pins as you go. Make sure the entire time that there will be enough space for the pipe to go through.
Guide one piece of pipe through each hollow tube you just created along the two opposite sides of the fabric.
Bend the pipes so that the fabric curves over the bed, with one pipe arching over the head of the bed and the other over the foot of the bed.
Secure the tent frame in place however you best see fit. Whether this is with string, heavy-duty tape or some other means will depend on what kind of bed frame you are dealing with.