If I have two candy bars that are the same size, I am sure the kids can pick which one will float and which one will sink.  And they will probably be right.  But what happens when I use a really big 3 Musketeers bar, and a really small Snickers bar?  I’ll try it tomorrow and let you know.  But, I will be very surprised if they understand that with density, size does not matter.  Only “crowded-ness” matters.