Tell me a polite way to tell someone to mind their own business in this situation.

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For context: we were doing a family art course recently. My child either had a toy snatched from him, or he left it and it was picked up by another child. Either way, my child pushed the other. I told my child he had to sit in time out for a couple of minutes, a) to think about what he'd done, and b) to calm down as he was wailing. He would then apologise to the other child.
Another mum approached my child and started telling him it was ok, he can play in a minute, there there it's all going to be ok etc basically wouldn't leave it alone. This did not help, it only made him start wailing even more! I don't know why she interfered. It did not help and we had to leave because she made him start screaming.
Is there a polite yet assertive way to tell someone to fuck off to not try to get involved when I am disciplining my child. He did something bad and was being punished with time out. He wasn't shouted at, no harsh words said. He was calming down until she interfered.

I always think that a well timed "please don't interfere when I'm teaching my child proper behaviour" goes a long way.

Is saying "interfere" to rude/confrontational, do you think?

I've had this once, beyond rude. I said 'we're fine thanks' in my iciest tone. Worked.

What crimbo said.

Perk up, smile brightly and say "Oh, do you want to swap? Which one is your child?" and rub your hands together as if you're going to walk over and start doing something with their kid.

I think interfere sets a tone that nobody wants to be associated with, so it's perfect!

@Sethis
I think that could be taken the wrong way!

It'd sure as hell get them to stop messing with your kid though, for sure...