You only need a few props to pull off an April Fool's prank on your kids.
Loosely stuff the pants with crumpled newspaper or rolled towels.
"Sit" the trousers in one of your kitchen chairs and use several pieces of packing tape to secure the seat of the pants in place. Or, you can thread the twine through the belt loops of the pants and tie them to the back of the chair.
Position the chair with one side up against the kitchen table facing the doorway.
Loosely stuff a pair of socks and fit them into Dad's shoes. Fit the tops of the socks into the pant legs and use tape to secure them to the inner material, if necessary.
Fold in half a piece of poster board and tape a few open newspaper sheets to it so that the newspaper extends beyond the top of the poster board.
Prop the board (newspaper facing out) on the mannequin's lap, pinning one corner between the chair and the table. Tape the lower part of the board to the chair.