To make the pattern, fold your pants in half and pull the crotch all the way out. Flatten them as much as possible and lay them down on top of your paper. Parchment paper is inexpensive and wide enough for most childrens sizes. I like regular rolled wrapping paper for adult pants since it's wide enough and it's cheap. Trace around the pants. If your pants have a drawstring or elastic, make sure you trace them with the waist stretched completely out. Add a seam allowance by drawing an outline 1 / 2 inch outside the traced lines along the legs and crotch and 2 inches along the waist and hem of the pants. Put the pants aside and cut your pattern out.
Fold your fabric just enough to accommodate your pattern. Pin with the longest part of the pattern right along the fold. Carefully cut around the pattern. Then do the same thing again so you have two pieces of cut fabric.
How you fold your fabric and where you cut will depend on the size pants you're making. With toddler sizes, you can use a mere 2 / 3 of a yard and fit both legs within a single width of fabric. With larger sizes, this isn't possible because the crotch makes the pattern too wide to fit twice. This is where a non-directional fabric saves you some money. You can flip your pattern and place it on the other side, sliding it down until the two crotches meet (boy, does that sound dirty) and waste less fabric