R.M.
I see two different issues here...first the email...IGNORE IT!!! Act like you never even received it. So what if she doesn't think that your son is going to have a good time....you know he is going to love it so don't even grace that email with a reply or a 2nd thought.
As to the money that she gave your son...I would have immediately taken than $20 from him...and said something to the effect "wow...how nice of Grandma to let you kids all pick out something that you want...we will save this until the next time we are ALL at Walmart". Act like you ASSUME it is for all of your children...I mean...what ELSE could she POSSIBLY mean??? Smile sweetly while you are doing this...and then I would figure some other way to get your son home from school on Thursdays!!!
Don't discuss the inequities around your children...they will reach their own conclusions as they grow up. My own MIL couldnt be bothered with my girls when they were little...she was only interested in "bragging rights" when they did something that she thought reflected on what a SUPERB family she had managed to gather around her. Now that they are adults....they couldn't care less about her...but it didn't bother them when they were growing up because we never made a big deal out of it and they had more than enough love and attention from MY Mom!!!