How about air? If it leaks its not a mess, and air compressors for home use are a dime a dozen. Put an air compressor in your garage, run a hard line to the bedroom, and put some junkyard truck airbags under the bed. Big lines, big valves, your kid will have instant "catapult" capability at will
Try
http://www.ridetech.com/forum . They can help you with the fundamentals of air suspension.