Avoid The Lecture

A lecture is only appropriate when the receiver is also intellectually engaged. It is not appropriate when the receiver is emotional, because reason and logic are temporarily disabled when upset!

Its not productive to explain the reasons we lost something. Acknowledge the upset first.

"Aw, you're really upset. It's a really nice stick! Its smooth with a nice bump on the end, and its bigger than you yet easy to carry. Its such a nice stick. And now someone else is playing with it and you feel like you lost the stick. That is really upsetting :( sad face :( "

and when the child feels like you're sympathizing, the screaming and anger will cease and then they'll just be grieving a little bit, and then you can nudge them in a positive direction, like, "If you loved the stick so much and then left it on the ground, maybe he'll do the same thing when he becomes interested in something else." And then, "Let's see what else could be interesting that might distract him from the stick." And then go find a bug, and show the kid with the stick the bug.

This way, the mother wouldn't be the bad guy. because its not her fault that the stick isn't available for play. Its just the way of the world. The mother becomes a source of comfort, instead of the harbinger of restrictions, which is an undesirable way to be viewed by an upset child.