Any ideas for special needs school playground party on concrete (eg cant have bouncy castle?!)?

Hello there, I have two autistic/disabled children who go to a special needs school. We have hired the school playground for my older boys 5th birthday next month. I thought we could hire a bouncy castle or ball pit but because the playground is mainly concrete no-one can hire us one.......I am really panicking now as we have a group of special needs children (age between 3 & 5) coming with their parents/carers and I cant just let them play in their own school plyground! Any ideas how I can make this a party? We will do some games but many wont understand.........The playground is quite big and there is a decked area and a small bit of grass. Please give advise if you can, I am so worried, thankyou.