Should fit, but depends on the rack width a bit. Fits on mine, with a fairly standard width rack.
One thing is it may be a bit less comfortable for the kid, cos the seat will slam into the rack on hard bumps, instead of using the full springiness of the mount stays.
Anybody used a hamax style seat on a bike that has a rear rack fitted too? Would both fit at the same time or do the feet bits fall in that area?