Seal any gaps in the toy pistol with modeling clay. Plug the barrel (if there is a hole there; some cheap toys will not have one) and fill the trigger-guard completely with clay.
Create a open-top box out of hot glue and acrylic. The box must be large enough to accommodate the toy pistol with about an inch to spare on any side and at least two inches on the vertical side. The toy pistol will fit into the box with the barrel pointed downward.
Glue two dowels onto the back of the handle. These will be used to suspend the toy pistol into the mold. Attach the other end of the dowels to a metal ruler.
Apply mold release to the pistol, the dowels and the sides of the mold-box.
Place the metal ruler across the open side of the box so that the toy pistol hangs into the mold-box without touching the sides.
Mix the components of your platinum silicon, then pour slowly into the mold box. A slow pour will prevent bubbles. Fill until the pistol is completely covered with at least an inch of liquid silicon on each side.
Wait for the silicon to dry per the manufacturer's instructions. Once the silicon is dry, remove the pistol and the silicon from the mold-box.
Starting from the dowels, cut along the sides of the mold until the toy pistol can be freed from the mold using an art scalpel. The side-cuts should follow a wave pattern to help fit the mold back together when molding. Do not cut further down than is necessary to free the toy pistol. Clean any remaining clay from the mold and apply a fresh coat of mold release.
Place the mold (without the toy pistol) back into the mold box. Mix a batch of expanding foam silicon and pour this into the dowel-holes. Use enough that the expanding foam expands back out through the dowel-holes.
Remove the foam pistol from the mold as you did with the original toy pistol. Using your art scalpel, cut away "flashing" (extra foam that expanded through the cracks) and clear the trigger-guard. Spray-paint both the foam pistol and the second toy pistol to match.
When filming a foam gun, cut rapidly between shots of a realistic prop pistol (the second toy pistol painted to look like a real firearm) and the foam replica. Only use shots of the replica when it is in motion.