Sandy, if you carefully compare the specs in the user manuals downloadable at the www.yamaha.com
site, it becomes clear that:
The CLP130 is basically a P120 portable, but with much better/larger amplifiers/speakers, and the wood cabinet of course, and the three fixed pedals (The very popular P120 only has two pedal jacks).
The CLP150 and the PF500 are both basically a P250 portable, with similar differences - cabinet and larger amp/speakers (watts of power and centimeters of speaker diameter). These models of course also add numerous voices and features.
The larger amp and speakers do help the sound, like a larger stereo hifi system has better sound than a table radio, especially true in the low notes in the bass. But you can add larger external amp/speakers to any of them. The portable P120 especially needs it.
The CLP120 only has one sound sample instead of the dynamic samples. Dynamic means they actually included three recorded samples of a grand piano when played softly, medium, and loud, and these three different samples trigger when you play soft, medium or loud. They call this dynamic sampling. All models mentioned except the CLP120 have dynamic sampling.
To that dynamic sampling, the PF500/CLP150 series adds a string resonance sample too, which is subtle, but it means that if you hold down one key, say C, and hold it a long time like a chord, and then play a lower C, you also hear the higher C resonate sypathetically too. Technically you should also hear the lower C cause this in the harmonically related higher E and G too (thirds and fifths), if those keys are down and undamped. Real piano strings do that, so they try to make the better keyboards simulate it too.
The kids are going to love any of those, for the real piano feel and better sound.