I’m dealing with a similar situation after making the mistake of being too nice and have also been searching for advice!

If you are able to install a privacy fence, it would make it more difficult for the kids to see when you are outside. You could lock the gates and/or have a rule that no one can come inside the fence without asking first. Any type of fence other than a privacy fence may not be helpful because they will either hang over the fence or talk to you through the fence and can easily see when you are outside. If you are unable to install a fence, landscaping might help create a barrier. You could use tall shrubs or a hedge row.

You could speak with the parents. Maybe explain that supervising several of their children in addition to your toddler is too much and ask them to speak with their children about not coming over as frequently. You are not their babysitter.

You could try creating boundaries by setting some rules for visiting your home and notifying both the kids and their parents. Maybe they have to ask permission before entering your yard, cannot visit on weekdays, have to pick up and put away all toys before they leave, and cannot dump sand outside the sandbox. If they do not follow the rules, send them home. Your house, your rules.

On days you allow them to play, you could limit the amount of time they can stay when they arrive. “Yes, you may play on the swing set, but you can only stay for 20 minutes and then you’ll need to go home because we are busy today.”

Maybe you could buy them a sandbox and some toys for THEIR YARD. Could be less expensive than a fence, but it’s possible they will still come talk to you when you’re outside. They may enjoy the attention from you as much as the toys.