Explaining Ionization to kids
I think the key concepts that are explainable at this age level are:


Atoms are made of protons, neutrons, and electrons
Protons have positive charge, electrons have negative charge, neutrons have no charge
Opposite charges attract
Atoms can gain or lose electrons, but not protons or neutrons
Atoms hold onto their electrons more or less tightly depending on which atom it is (you won't be able to really explain why, but you could just show them a periodic table and explain that non-metals (the right side) like electrons more than metals (the left side))
Atoms can "steal" electrons from each other
Electrons can be "knocked off" of atoms by high-energy photons (light or EM radiation)


The difficulty with explaining these things by analogy to regular-life experiences is that there is no regular-life experience that captures the whole picture. This means you are limited to activity-type demonstrations, videos/pictures, or analogies for specific pieces of the puzzle.

I would recommend a mixture of all three approaches. Kids like hands-on stuff, so maybe you could explain the basic concepts with very simple analogies, then do an activity where the kids are given "electrons" (tennis balls, maybe) and are assigned to be metals or non-metals. Then you could have them give each other electrons and count charges as they go. If you had containers that could hold 8 electrons, you could even represent valence shells and the octet rule - metals could start with two or three electrons, non-metals with 5, 6, or 7, and the metals would need to empty their shells while the non-metals would need to fill them.

This would demonstrate a very important concept that is hard to teach in the abstract: Electrons don't disappear.

Since you are also interested in teaching about ionization by radiation, you could assign each student a number representing their ability to "hold on" to an electron. Here is where you might be able to use a shopping-type analogy - only instead of students giving up electrons as money, they would give them up in exchange for money. In this case, the money represents the energy, and you would be giving them the energy (money) needed to remove (sell) an electron. Atoms that charged a higher "price" (require more energy) would wind up with more money at the end of the exercise.

There are many variations on these ideas that you could use, but I think anything that covered the seven points I mentioned above while simultaneously developing an intuitive understanding that energy and electrons are conserved could be called successful.