It might be fine with either 5 or 7 volts, but if you’re worried, search online for 12v to 6v DC converters. You could put two of the battery packs in parallel

It might be fine with either 5 or 7 volts, but if you’re worried, search online for 12v to 6v DC converters. You could put two of the battery packs in parallel (one set to 5v, one set to 7v) to get 12v input. Then connect the 6v output of the converter to the baby monitor. There are probably other converter options that would work with just one 5 or 7 volt input but 12 to 6 is very common.