You could most likely get around the feedback issue by routing the mic and speaker through a DSP and then EQing out the sing frequency. This makes the system full-duplex and allows for the client to scream at the kids and hear the replies without any action on the kids' part. That's going out on a limb, I've only used panel-to-panel intercom a few times, and the talker was only about 36" away.

Any phone hybrid (nexia TC, XAP TH2, Converge TH20, Polycom Vortex) could be used to integrate the panel system with any PBX that provides a loop-start FXO port. Clearone has a VoIP hybrid called the VH20; I get to play with a few in the coming months! The macros and dial plans would be tricky. The way I see it right this second, you would either need one hybrid per panel to give each panel its own extension, or have one hybrid per system and only be able to call "all panels". Verrryyyyy expensive... We're talking tens of thousands here.