The kids next door let themselves through my side gate to retrieve balls. Is a lock an overreaction?

I was washing dishes when I saw a child run past my kitchen window.

For one startled second, I thought someone had come through the back door. Then I saw a soccer ball under the hedge and realized the boy from next door had opened our side gate, crossed the patio and collected it. He waved at me through the glass and left.

This has probably happened more than once. Balls land in our yard regularly, and they often disappear before I toss them back. I assumed the children were reaching through the fence or that their parents collected them when we were out. Now I know the gate has become part of the retrieval route.

The children are about eight and eleven. They are polite when we speak, and the parents are friendly. I do not care about the balls. I care about people entering an enclosed yard without checking, especially because we sometimes let our dog outside. The dog is gentle but excitable, and I do not want a child surprised by him or leaving the gate unlatched.

The gate has a simple latch that can be opened from the outside. I could add a small lock this afternoon. That would solve access and might send an unnecessarily cold message if I do it without speaking. It would also mean we need a key whenever we move bins through that side.

I could talk to the parents first: “Please have the kids knock before coming into the yard. We are happy to return balls.” That feels completely reasonable. My hesitation is that the parents may answer, “Of course,” while the children continue doing what is quickest. A physical boundary does not depend on memory.

There is also a tone problem. I do not want to lead with the dog as though I am threatening consequences. I want to explain why the gate must stay closed and why an adult needs to know before anyone enters.

Yesterday another ball came over. I left it by their front steps and did not mention the window incident. I have been rehearsing the conversation since, which is usually evidence that the actual conversation should already have happened.

Would you speak first and add a lock only if it happens again, or install a latch they cannot open and then explain? Is there a friendly way to make “do not enter our fenced yard” sound as ordinary as it should be?