Indoor games in 2 teams of 4 such as passing a balloon back over heads, back person runs to the front with the balloon, when all have had a turn and are back in original places first team to sit down wins. Same idea - pass the balloon back between legs. Doesn't need much room.
Musical statues, freeze when the music stops.
The Keys of the Kingdom. Sit in a circle one person is blindfolded and has a bunch of keys on the floor behind them. Adult chooses someone who has to walk silently right round behind everyone, 'steal' the keys, tiptoe back to their place and sit down. If successful they are the one blindfolded next.
While they try to tiptoe around the blindfolded person points to where they hear a sound. If they point correctly the person trying to take the keys must sit down and someone else is chosen? There are lots of similar circle games often used in drama sessions or cub/brownie meetings.
Zombies. Sit in a circle a zombie is chosen and a name called out by the adult. The zombie, arms outstretched, must walk slowly towards the named person. The named person calls out someone else's name so the zombie makes for them instead. Anyone reached and touched by the zombie becomes the zombie next.
All would are normally for larger groups in larger spaces but, I think, worth trying for active 9 year old boys indoors!
Hope they enjoy the party, hope the weather allows them to enjoy it out of doors!