I am merely looking for a piano replacement that will let them play with some synthesizer-like effects. But keyboard action is the key.
Still doesn't answer the question. Do you need a piano weighted keyboard? Or just any old "good" keyboard? Because there's a huge difference in cost. Maybe ask the girls if they're bothered either way.
The cheapest piano weighted controllers are probably those from CME. They're a newish company in the West, made in China for cheap, but have a pretty good rep for reliability and build quality (the phrase "built like a tank" comes to mind). Yamaha Roland and Korg probably make some too, but it's not something I've looked into for a while, and they tend towards the expensive (as in upper hundreds into thousands).
Off the top of my head, the best cheapish ($100-$500) and readily available second hand keyboards as far as the actual keyboard action goes (non-piano weighted) are:
Yamaha DX-7
Yamaha CS1X, CS2x, AN1X, AN2X
Roland D series
Ensoniq Mirage, ESQ-1, EPS series
Out of those, I'd probably recommend the Yamaha CS/AN series. Good keys, good rep, range of knobs, nifty internal sounds with editing. The AN does analog synth modelling (ie DSP simulated analog synth noises), the CS more like a General MIDI soundset, but with good tweaking options. You can probably pick up a Roland D50 for next to nothing, as no-one much cares for their internal sounds these days. Good keys, no knobs.
But you could also probably pick up a decent new or used dumb controller (ie no internal sounds, just a keyboard and a few software assignable knobs, and all sounds generated in the Mac) from M-Audio, Roland/Edirol, Yamaha or a few other manufacturers who escape my mind at the moment. Check sonicstate/studio for reviews. (Also $100-$500 with $200-$300 being typical, new with warranty and a software bundle)
does it matter whether the thing has a usb connection? Or do the midi-usb converters work just fine?
Nope, and yep. The only difference is the extra cost of a USB-MIDI converter, which you could otherwise roll into your keyboard budget. Some of the kbs with USB
and MIDI ports can also act as an interface between your computer and other MIDI gear, should you decide you need any later on, but that's probably not a huge priority for you.
All of the above is off the top of my head and I encourage you to confirm or refute it with your own research. I hope it gives you a useful start though.