Ah, the world of the Part 15 Radio Service. You've settled into a time and dimension with the potential for being the "wild, wild west." Those handhelds were most likely licensed under Part 15 like Citizen's Band (CB), the General Mobile Radio Service (GMRS), Remote Control (R/C) devices and toys, and a whole host of "unintentional radiators" like scanners and computers. Since you're a lawyer I'll send you to the rules via the shortest URL possible. Go to http://www.fcc.gov/oet/info/rules/ and click on "Part 15." Scroll down the list and you'll see not only the rules but the frequency bands that the baby monitors and the walkie-talkies can operate on legally. On the other hand, if the radios don't say that they're Type 15 Accepted (in the instructions or on a tag on the radio) you may want to contact a lawyer specializing in 47 CFR Part 15

As you scroll down Part 15 you see a daunting frequency list. If you have a scanner you could get lucky and find the frequencies quickly. If you know any amateur radio operators you can ask if they know anyone with a frequency counter or a spectrum analyzer -- they'd likely be curious to find the operating frequency, too. Newer equipment is likely to be higher in frequency (900 MHz and up) and harder to cover in a scanner. I only have one scanner that goes up to 1.3 GHz, for example.

The oldest baby monitors use a short list of possible frequencies. A Google search didn't reveal the Rescue Heroes frequency but some curious person may discover it and post it.

I'm impressed that you got them walkie-talkies. My appreciation for shortwave listening and two-way communications led me to get licensed, which contributed to me getting involved in my career in public safety. The internet can make us take even reliable and long-range communications for granted. Nice to see that kids still have access to real-time communications without a per-minute fee.