How to invite both divorced parents to DC birthday party?

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So I have DS soon to be 6 and DS soon to be 1 joint birthday party in around 6 weeks, there birthdays are 3 days apart so we have decided on a joint party this year.
I want to throw them a decent party as it's a joint one, maybe hire out a restaurant buffet type thing however my parents had a messy divorce when I was younger and they do not speak, DM and my Gran also do not speak and my step mum would have to be there and they also do not speak.
Is the whole thing pointless and would it cause too much tension?
Is there a way around it?

Is the party for family or kids?

I'd do a kids party just for their friends.

Honestly? Don’t have a family party. We don’t for similar reasons.

At 6, your ds1 may prefer a party with friends. If the family doesn’t like it tough: it’s a party for a 6 yo, not for them.

I agree that the party should be for the children not for the family. The 6 year old would want his friends there, and the 1 year old won't know.

Don't invite adults who may behave more childish than the children. Agree with pp that it's a kids party, so invite kids and have a lovely time. You may want to do a mini birthday tea another day for DGM or DGP ( not together) but let your children have their happy birthday party and enjoy it. You'll have enough to do on the day without worrying about their petty battles. We stopped inviting DGP to the DCs party past age three because the kids don't care and we're too busy to engage with them which made them huffy so now we do separate little things and leave the party for the kids to enjoy.

At 6 I imagine your dc will massively prefer a party with his friends doing kids activities than a buffet in a restaurant.