some pro's and cons (just as i see it!)

usb phone to phone is out - no host mode and the custom cables required would be a killer.
usb to pc - possible, but not the simpleat or most elegant solution, plus kids lose their cable ten mins after opening the box.
bluetooth - OP doesn't want, so forget it anyway.
web server - possible, entirely software based, more elegant, though requires web server (duh) username, pass, etc. plus requires kids phones to have internet enabled (depends on your views as a parent on unrestricted web access) and also privacy issues
email - some privacy issues, should require manual download of contact file so not fully "polished"
text - could be made to work, fully integrated solution, however would require installation on BOTH phones.