You might be misreading the original post.
OP goes to park w/ kids
A maskless child approaches him/his children
Due to OP's feelings regarding masks in a pandemic, he tells the child (presumably nicely) that they can't play with his kid(s) without a mask
This SHOULD be the end of it, for any reasonable parent. This was her chance to go "whatever, you don't HAVE to play with those kids" to her own child.
Now, likely 1 of 2 things happened in the next part, which is part of where OP maybe could have handled it more delicately, for me.
The child either:
Went back to their mother and told her verbatim what OP told them - you can't play with my kids without a mask
OR
Went back to their mother and told her something closer to "the man said I need a mask to play"
See now, I do think #2 IS more likely - kids are simple. They see it all simple. It's this equals that. I can't play with the kids means I can't play at all, cuz that's the play I want.
If the kid went route 2, yeah maybe Mom is a bit justified in her mind saying "hey what the hell, how dare you tell my kid he needs a mask!" as that is not the rule (kids under 4 blah blah blah).
All that said, from OP's perspective, it basically ended at #3 waaay up at the top there, then suddenly he was confronted with an angry and seemingly unreasonable woman.
That's my takeaway.