Brief history: Pils are a world apart from me (what I am about to say may sound snobby but imagine they're your ILS), they read The Sun, talk about tit sucking (breastfeeding), FIL comments on women's breasts to my dH in front of me..... from the off say lovely things about DH's ex gf, argue and have a go at me ain a way they wouldn't to DH and usually when dH wasn't around. They used to turn up Thursday for a weekend visit and leave Tuesday and treat the house like it was Dh's only and I was a guest, more tha them. The list goes on. Children come along and FIL teases and is rough with our small babies, MIL constantly picked at what I did..... NEVER at DH only me. FIL helped DH with house stuff and effed up loads.
So many many small shit things to build an uncomfortable relationship. They think I'm a snob and high maintainence and I think they're rough and inconsiderate. WE don't like each other. It all came to a head when MIL reacted to something I wrote on FB about my own mother, saying something nice. She decided it was really a dig at her, which it wasn't. She responded with a 'Joanna, now there;s a girl you would really want for a dil' and I overheard her bitching about me and my mother on the phone, it also transpired she had been phoning DH at work telling him my status updates (he isn't on FB).
So very petty, but a truly 'over' relationship.
They are now asking to come to dd's birthday party and I don't think I could be in the same room as them. There's no reason for them to come, her birthday is on Tuesday (DH is at work and so no way they can come then, I'm ion Bristol and they're midlands) and dd's party is at the end of the month as she's five and starting school so no friends yet. They can see her in between, the party is a chance for dd to build friendships.
I think - I wouldn't want my children anywhere near them. Until the children are old enough to have formed their own opinions, because there's no way I'd want the grandparents passing their own opinions and ideas on.
I would also be very unhappy about my children spending time with people who obviously don't like me very much.
What I would do is point out that their past behaviour has made it impossible for me to have any kind of civil relationship with them and therefore made it impossible for me to facilitate their relationship with my children. They would not be welcome at any birthday parties and quite frankly I wouldn't want my DH taking them up to visit without me either.
If the children want to know their grandparents when they're older (i.e. teens) then I might provide contact details. Maybe.
No they cannot be at DD's party. Why would they want to anyway unless its about THEM as your MIL already seems to think about lots of things. They have been repeatedly rude to you as DD's mother.
I would tell them, and tell them why (the rudeness and insults to you) without getting into an argument. No need to tell them any more. If they want to do a birthday event with DD, they can do with with your DH another time.
Do you think they have an idea that your mother and sister are coming? Could this be why they now want to come as well? Still NO.