There shouldn't be any problem with air shocks...I'd have used them myself probably, if you like to carry loads and go travelling.
Something's hitting something, or broke. You might have to get under there with the grandkids jumping up-and-down in the trunk and find what's going on.
edit: Also be sure the shocks are somewhere around the middle of their travel at rest, and don't bottom out before the bump stops. You likely have to take one out and measure it at extension and fully compressed, then compare measurements at the suspension mounting points, in order to do that. Of course air shocks add some load to the mounts so they'll need to be tough enough to take it.