Next doors child always in my house

I feel for the little girl but it’s you who is letting her in and it’s you who is supplying drinks and snacks.
If you keep allowing it then it will continue. Maybe if you consistently sent her back to her great grandparents each time, they’d be telling the girl’s grandparents that this isn’t on. Worth a try.
You can explain to your son that she’s supposed to be seeing her family. End of.

Stop being a wet lettuce and send her back next door.

I don’t understand why you let her in.

There’s a doormat machine somewhere churning them out.

“No, Katie, you can’t come and play today. Or tomorrow. Or the next day. I don’t mind you coming over occasionally but you are here to see your grandparents. Back next door you go. They can feed and entertain you.”

And that’s why you’ve ended up as unpaid childcare - because ‘No’ is always tomorrow!

She's a kid , you don't have to worry about offending her. You can even say stop with the questions . And sorry you have to go now / can't come in now , etc. No need for explanations .

Do they play outside ? Tell them to get out in order to give you space. Have drinks if necessary already made up on fridge .

YANBU OP but god, this poor little girl, how sad that none of her relatives can be arsed with her. I feel really sorry for her.

There have been so many threads like this over the years and I honestly cannot muster up any sympathy for martyrs who just won't say no, and stick to it.

You're a grown woman OP 🤷‍♂️

in that case, she can say "she can come when I invite her"

Then she would never visit again. I don’t think OP’s DS would be happy with that.
There is no issue with having boundaries and days when you are not available.
At age 7 and 9, I wouldn’t think they’d need much minding, I’d limit the days but wouldn’t ruin the friendship.

You need to explain to your son that it’s not happening today.

I would go round to the great grandparents and speak to the grandparents when they drop and tell them to stop using you for childcare.

"Because I don't want that today"
You could also say to the grandparents that you cannot be responsible for someone else's child and can they please put a stop to the visits.

In many ways, you’re not helping the situation by allowing her to come in and play. Next she knocks, tell her it’s not convenient i
and send her ‘home’. If your son asks why, you could tell a white lie and say you’re going shopping, even if you don’t’t actually make it out the front door, or say you’ve got stuff to do, and unable to supervise, or just say she’s here to visit her grandparents, so needs to spend time with them

He’s obviously enjoying the company. Kids are annoying at times, that’s life.
Just set boundaries, the summer holidays flies through, if they enjoy spending a few hours together, let them.
If she asks for things tell her to ask her DGP she’ll soon learn.

he's old enough to understand that mummy does not want to host a playdate.

They're not mandatory.

Just say not today, on the other hand I always enjoyed having children around when mine were small, they were both awkward and sometimes friendships needed extra tlc to sustain them.
I didn’t mind it through the summer holidays. Now they’re 17 and 12, those days are a distant memory.
On the days she is allowed come, tell her to stop asking for things, put out a small table with a jug of juice and a few snacks.
It’ll help your child build confidence too.

I guess it’s not really the little girl’s fault - after all, no child is going to want to sit in an old grandparent's house when they can be next door playing with toys and their friend.

But yes, you’ll just have to be firm with the neighbours. You could perhaps say she’s welcome to come for an hour before lunch but after that you’re taking her back, no exceptions. I feel a bit sorry for the kid - sounds like her parents, grandparents and great grandparents can’t be bothered with her. But that doesn’t mean you have to pick up what they’re not willing to do!

Instil some boundaries in both of them. It’s not cruel to say no. You can even say to your DS that she comes to see her DGPs and by coming over to yours everyday she is not
spending time with them.

Parents call the shots, not DC or other people’s DC. It really is quite simple.

So you tell him that Katie has to spend time with her grandparents, and that you and him and doing X or Y instead. Or you revert to the tried and tested "because I say so"...

Don't let her go inside? Just tell her you are busy and it's not convenient.

You are the one who let her in, you are a bit silly to be annoyed by it. You can be polite and friendly, and send her back to her grand-parents.

It's not a home invasion is it.