I remember this hardback white book from my childhood. It had a lot of stories in it. The one I remember most vividly was about a club of boys and girls who did all sorts of stuff. They host an event where they come disguised as monsters. They figure out who everyone is, except one monster who they assume must be the last club member (I think it was a girl, Katie?). So this yellow monster doesn’t look like a costume like the others. The monster ends up winning lots of events. But then at the end, the missing club member shows up late and it turns out it was a real, friendly monster the whole time. Another story in it was about a girl who received wishes for her birthday? I think from her aunt? It’s possible she made these wishes using candles? Or a magic bag. The wishes all go wrong. I think one makes these awful ugly babies or monsters? And one involves cr
The story with wishes might be “All You’ve Ever Wanted” by Joan Aiken. https://joanaiken.wordpress.com/2020/09/03/all-youve-ever-wanted-a-joan-aiken-wish-comes-true/
She’s written a lot, so I don’t know if the other was one of her stories, or just included in an anthology.
The story about the girl who receives annual birthday wishes from her aunt that always go wrong sounds like Joan Aiken’s short story ‘All You Ever Wanted’, which is, I believe, included in a number of anthologies. I don’t recognize the other story.
There are some similarities with Linda Neves’ short story ‘A Simply Monstrous Time’. You can find it in ‘Martin Composes a Tea: and other humorous stories’. A party, a shy girl (Isobel), monster costumes, underneath the monster costumes are more monsters.
https://archive.org/details/marvincomposeste00hunt/page/84/mode/2up?q=%22Linda+neves%22+%22monstrous+time%22