The average density of the human body is slightly higher than that of water which is 1000 kg/m^3. Thus a 20 kg child would have a volume of about 0.02 m^3 and 600 children of similar size will take about 12 cubic meters of space, or something like 2 m x 2m x 3 m. That's a very large container, but not extraordinary large. Now one must factor in the air pockets between the bodies, which may be up to 20% of the space or more.