My 12 year old son stole $30 from my 8 year old daughter. How do I handle this?

So my son came to me this morning with $20 and he said he wanted to order a new computer mouse on Amazon. I asked him where he got the money and he said he found it in one of his pockets. The mouse he wants is $40. I told him he would need to work for the other $20 this weekend. A short time ago, my 8 year old daughter came in my office crying that $30 is missing from her desk. Pretty obvious what happened. Any advice on handling this with my son? I obviously need to talk to him about stealing but I am worried it will go over his 12 year old mind. via /r/Parenting http://ift.tt/2nsmhZX