If you are adopting from foster care, you'll likely have safety issues you would not ordinarily have. That being the case, you can do alarms on the bedroom doors, room monitors/video and audio. It's not spying, it's making sure everyone is safe if there is any issue at all. In fact, I would venture a guess that a social worker would be asking you how you'll keep an eye on the kids and recommend the monitors themselves.
Even without the foster care element....kids can sleepwalk, have nightmares, get sick, etc. and a monitor is a great way to be aware of these things so you can answer their needs in the middle of the night.
If you like the house...get it.
