[gred]

We used a similar setup, but opted for two sets of analog walkie talkies (instead of one set), so that we could fall back to the secondary set if the batteries ran out in the first set (we replaced the batteries in the two sets of walkie talkies at one-month offsets so that they wouldn't run out at the same time). We also had the two sets running on different channels, to avoid concurrent outages caused by unexpected radio interference. We did the math afterwards; overall availability of the child monitoring system was four nines. Satisfactory.