A birthday party is designed to be loud, bright, crowded and unpredictable, which is precisely the combination that overwhelms a lot of children. None of the fixes require cancelling the party.
Skip the loudest music, or keep it low. Avoid flashing decorations if you have a choice. If singing happy birthday is part of your plan, warn your child it is coming and let them opt out of being the centre of it if they would rather.
One room set aside, known in advance — "if it gets too much, you can go to your room, that's always allowed" — does more to prevent a meltdown than almost anything else on this list. Permission to leave, stated before the party starts, lowers the pressure of the whole event.
Cake at this time, presents at this time, games at this time — the same predictability that helps on an ordinary day helps more, not less, on the one day everything is different. A child who knows the shape of the party can pace themselves through it.
A smaller party is not a lesser party. If a large gathering is expected for family reasons, consider a separate, smaller celebration with close friends as the one that is actually for your child.
Only if you want to. A quiet room and a shared schedule work fine without any explanation at all, and you do not owe anyone one.
Let them. Do the cake without the group singing, or open presents later at home. The party can be rearranged around what actually works for the child it is for.
Yes. A party sized to what your child can enjoy beats a bigger one they spend mostly trying to cope with.
Part of our guide to Big feelings & belonging.