My grandchildren's birthdays are 2 weeks apart. Last year, when they became 10 and 7 their mother had one party and it worked well. It was a swim party at a community center. Each child could invite several friends. Relatives also were invited. She served cup cakes. The center tried to play games with the kids before they went swimming but the kids just wanted to swim.
Of the several friends my grandchildren invited none showed up for my grandson and only two for my granddaughter. There was only one response to the RSVP. The mom said they'd be there but didn't show up. Go figure! Disappointing but because there were cousins and younger neighbor children there both grandkids had fun.
Because kids didn't show up my daughter has told each child that they can invite 2-3 friends and they'll do what the child would like to do. My grandson wants to go swimming which they'll do at a friends house this year. My granddaughter wants to go skating. We'll do the cake and ice cream away from the pool and rink. Haven't decided about that yet.
The year before my granddaughter had a swim party at a community center and had 3 friends. We were going to have cake and ice cream at the park but it rained. We had some sort of goodies, I don't remember what, in my van and opened gifts and handed out "prizes" without playing games.
My van's seats fold down to make a large open area. The girls loved it and asked to do it again. It was just the girls and me.
We went out to dinner with family for my grandson's birthday. He wasn't much into friends then.
It seems to work to have a party together because the kids split up anyway based on who is whose friend and family goes back and forth between the two groups. It does mean having an activity for which that works. Another birthday we celebrated both by a bar-b-cue at the park. My granddaughter had friends but my grandson was young and didn't care. Grandpa played games with the girls and my daughter entertained him and family.
They always share a cake and don't seem to mind. But then they get cake periodically throughout the year.