I'm from Mediterranean culture but have lived in the UK for over 15 years and have two DC.
We've been going to birthday parties for years now and they used to be very relaxed, fun and a nice way to celebrate the birthday child
In recent years everything has become so rigid and I'm wondering why... Sorting food is the most stressful thing ever, party decorations take ages to do (I blame social media expectations for this) and bringing a sibling seems to be SO frowned upon. Many parents don't have another mean of child care so leaving one child at home isn't an option for them. Most of them are also happy to pay for the additional child's soft play or venue ticket.
But why the nastiness and talking behind people's back (I can't believe she brought her other child.....)?
I'm very much it's a kid's party, come have some food/coffee and let's chat while the kids play.... But it's obviously cultural difference.
I wish we weren't so rigid here, we pass that behaviour onto our children and a mark on the community that people aren't as welcome as we want it to look like.