I can feel this for you.
7th birthday. We make our invites on the computer. I give DD 10 invites to bring to school and pass out. Unbeknownst to me she goes onto computer and reprints 10+ more invites and pass them out to school. Sudden we're getting calls from everyone. The count 25 girls.
Now I'm planning a luau party, we have a large swingset and a pool, plus our yard is big enough I figure 25 girls and their parents, no problem.
NO - it rains like it's never rained before. The gutters are flowing over, the sump pump is going off because the ground water is rising too high.
The party moves from outside to my cellar - thank goodness I have a large open room, not a pretty one mind you. It took my mother and I all day to redecorate the room with Haiwian themes. Never again.
Now I say you're 9 - you can invite 10 people. You're 6 you can invite 7 kids.