Do you have an existing system that you are trying to monitor, or are you doing research to see what would be best for your situation?
There are a couple methods, but the baby monitor specific route is more expensive. Of course theres no price you can put on a baby's well being, but I prefer the alternative method. If you buy a baby monitor that has an app, the video feed will be exclusive to that app, and you have to pay a bit more for additional cameras that are matched to the frequency. I prefer using a wireless IP camera.
You can set up any wireless IP camera with sevaral apps that are universal across all smartphones and tablets. They are secure video feeds and offer more options. You can link multiple cameras together and even use cameras with built in pan and tilt. My brother has a setup that he can monitor the baby in the crib with infared lighting while he sleeps. If he rustles, the app can alert him. He can also speak through the two cay communication built into the camera to calm the baby. Works well with the cats while he's away too. He has four cameras that he can control remotely to monitor the entire house. I've been thinking about doing this myself. One app that comes to mind is IP Cam Viewer. Most wireless IP cameras will let you know what the recommended app and compatability is. I would do some research on the cameras that have the features that you are looking for and the associated apps.