My mom cashed my kids child benefit check into her account without my knowledge?

As others have said, call CRA and discuss with them the fact you didn't receive the check.  DO NOT mention that it you suspect it was your mom or you think you know who it is. They may be more likely to blow you off and make you go deal with on your own. "I did not receive nor did I cash the check. I need to know what to do in order to get the money which, since you said it was cashed, was stolen and fraudulently cashed."  CRA can trace where it was cashed.  They may also tell you to file a police report.  Follow their lead and push them hard if they try to blow you off.

Yes. Mail theft has a hefty penalty because it's federal. 

Did you owe her money or is she just a klepto?

Debs has best answer

your mom is going to lose her house

Your mother committed fraud. Even if she believed you owed her that amount and cashing it was a way of getting the money back (hypothetically), she still committed fraud. I would firstly report it to the department who issued you the check. Ask them what action you need to take to recover it. Either they will handle the situation or suggest you report it to the police. Warn your mother that this is what you intend to do. She may think twice and give you the money, though she has probably spent it by now. 

The check was issued  by CRA, tell them the check was not endorsed or cashed by you and you will have to sign a paper stating you did not endorse or cash .  They may require you to file a police report, but you have to start with the check issuer, not the bank.

Well, I would confront her first and ask for the money, but yes you can report her for theft.

Call back CRA and report fraud...your mom will go to jail for committing bank fraud.  

You wont see that money anytime soon...keep that in mind.  

Are you 100% sure you dont have any shared accounts with your mom?  Even one you might not know about?  

Contact the bank, your Mother must have forged your signature.