It's tempting to geek out on the answer, but the below clip summarizes the gist of my explanation attempt. (No doubt we'll be returning to this one!)
As soon as we arrived the children discovered a bunch of balloons left over from an easter egg hunt, and proceeded to experiment by tying a basket to them and adding and subtracting balloons to see how many were needed to get the weight to rise up.
Having succeeded in this, they then encountered the problem of getting the balloons back down from the ceiling:
A number of recovery attempts were made, including an audacious one by our friend Ben involving use of the remaining balloons to lift a string connected to a clothes pin held open by a melting ice cube. (Ultimately the children had success with a simple ladder).
By now we had made some comments about air and gases, and why helium made the balloons rise up, in contrast to the plane.
On the second morning at the pool one of the girls asked, "What is water made of?"
This launched a discussion about the elements, molecules and atoms. The children enjoyed asking each other what various things were made of (wool? smoke? another child's ear? the sun? pee?)
Until E asked "If everything is made of atoms, then what about the atoms?"
At this point we watched this animated video about atoms. (There's an explanation of how hydrogen atoms bind to oxygen to form a water molecule at the end).
We then spent the afternoon at Griffith Observatory on the Hollywood Hills, which is filled with models of celestial bodies – strikingly resembling the atoms from the morning.
Tomorrow we'll pick up from where we left off, and start by watching The Powers of Ten, one of my favorite science film classics. We also use The Physics Book: From the Big Bang to Quantum Resurrection and other resources in the classroom.