How do I stop one pool invitation becoming permission for neighbours to drop their children off?

My neighbour texted, “The boys are ready—can I send them round for twenty minutes?”

I had invited her family to swim last Sunday. She and I stayed by the pool, and it was a pleasant afternoon. This text arrived on Wednesday while I was cooking dinner. I hadn't offered another visit or agreed to supervise her children.

I said the pool wasn't available. She replied, “They can both swim. You don't need to entertain them.” I wasn't thinking about entertainment.

We live in the US, and the [CPSC's drowning-prevention guidance](https://www.cpsc.gov/safety-education/neighborhood-safety-network/toolkits/drowning-prevention) recommends close supervision alongside barriers such as self-closing, self-latching gates. Our gate is shut, and I haven't given the children a way to enter independently.

I want to keep occasional invitations possible, but her answer makes me think she heard “family swim” as “the boys can come over.” How do I reset that before another sunny afternoon? Saying I am busy today doesn't explain that an invitation and an agreed supervision plan would be needed every time.