i have been asked to make a bmx themed birthday cake. It has to relatively simple yet fun as it is only a small cake and the client doesn't want to spend a huge amount

I have no insight into bmx's. The client said the child likes freestyle, jumps and stunts!

Any help would be much appreciated

How about something like this:
http://www.coolest-birthday-cakes.com/coolest-motocross-bike-birthday-cake-15.html

The baker even describes how she went about doing the decoration!
Good luck!

Do you know what their bike looks like? Freestyle and stunts for bmx riders usually include pegs on their bikes so they can grind railings on curbs and stair railings. so maybe a set of stairs with a bike on the railing? cant wait to see what you come up with.