So, purely from a historical perspective, we want to summarize the metals used in the manufacture of old toys. If nothing else you need to go away knowing which metals were used during which time periods -- this will help you a great deal in estimating dates on collectible metal and tin toys.

First Metal Toys Were of Tin or Tinplate
Tin or tinplate is simply thin sheets of steel covered with tin. It has been used in Europe since the late 18th century to make kitchenware and small metal objects, including some toys. It's first use in America (for toys) is dated around the  1820's and was used only by tinsmiths.

When tin ore mines were opened in Illinois, in the 1840s, local tin ore became available and American manufacturers gradually began to apply European innovations, such as the mechanical press for stamping metal into kitchenware and, subsequently, collectible toys. 

By early 1900 1/3 of all metal and tinplate toys made in Germany were sold in the United States, but early in the 20th century American firms began producing large quantities of spring-driven tinplate toys.

During World War II, the production of tinplate toys was discontinued because of the scarcity of raw materials, After the war, large numbers of these toys were produced by Japanese, Korean, and even Chinese firms. However, by the 1970s metal, tin and tinplate toys ceased manufacture -- the material was basically replaced by plastic.

Another great reference on antique metal toys is Antiquetoys.net.

The Most Popular Metal in Antique Metal Toys
American toy makers excelled in making cast-iron toys! It was easy to mold and mass-produce (the American way) and could be used at modest costs. The use of cast-iron in toy making hit its peak in America during the second half of the 19th century. European makers had been using iron for certain toy components like wheels since the 18th century, but never exploited the full use and advantages of the cast-iron material.

Cast-iron was replaced by other cheaper metals but continued to be used to make toy trains, trucks, cars, planes, and the like until World War II. 

Lead Antique Toys
Most early American toy soldiers were made of lead or other lead based alloys (like pewter) -- few other toys were ever manufactured using lead.

Sheet Steel Toys (Heavier than tinplate)
Steel has been used for centuries in weapons but did not make its way to the toy industry until about 1850 in America. It was a very latecomer as an antique metal toy making material. Initially it used only for small parts requiring extreme durability. 100% all steel toys began to be made around 1900. The material (because of its strength and durability) was used almost exclusively for all the rideable toys such as bicycles and miniature walk and petal cars. These were of course followed by the super-heavy-gauge steel trucks and and cars eventually made by the Tonka Toy Company.