Keep in mind that lead is rather toxic. It is not suitable to make a toy to be handled frequently. If you can please use something like pewter instead, it also has a low melting point. If you absolultely need to use lead don't melt and pour it inside. At the very least go outside.
Plaster can withstand heat, but in order to get decent results it needs to be really dry. And really dry means not just air dry. You probably would have to put the mold into an oven. Start out with less the boiling temperature of water. When you think it is dry (the time will depend on the size of the mold) heat it up to maybe 300-400F to make absolutely sure no water is in there. Be careful when you get it out of the oven not to burn yourself. Ideally you would pour the metal into the mold while the mold is still warm.
If you mix silica sand into the plaster it will get a bit more sturdy. Actually a mix of silica sand and plaster (e.g. 1:1) is used for molds for metal casting, even for metals which get much hotter than lead.
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This content was originally posted on Y! Answers, a Q&A website that shut down in 2021.