Would you consider running it yourself, ginx, or roping in a few older cousins? For DS's last birthday (9)in December, we took him and a group of friends to our local fields armed with torches and glow sticks at about 5pm and played a variety of capture the flag/hide and it style games. Then came home to a fire pit (old BBQ) in the garden and made s'mores. Boys went home saying it was the best party, ever! Cost about £40 for the glow sticks.
We did something very similar for ds's 11th last November. We went to the woods in the mid-afternoon but it was still relatively dark by the end! They were in teams - did den building, capture the flag, 50/50 etc - we found a site with a stream through the middle so that added extra interest. They then had a massive nerf gun battle (only hassle = collecting the bullets afterwards but we said they could keep the ones they found so that helped). We bought them backpacks from ebay for £1.50 each and they carried their nerf gun in that, whistles from ebay (10p each), nerf gun bullets also from ebay (500 for about £14) and some tarpaulin and rope for the den building (which we then kept). I also bought camouflage fabric and cut up strips which they tied round their heads and we used camouflage face paints. We gave them "rations" which they had to carry and we marched to the woods and then onto the car park at the end. They were extremely
muddy (I got brownie points for bringing black plastic sacks for them all to sit on in their parents' cars!) but all said what a great time they'd had. I think some of them had never really gone for a muddy walk in the woods before, let along played in them!