Well, first of all, nothing is impossible. But, you are likely ok. However, the risk with the 5 volt is more likely that it just won't power up and function a

Well, first of all, nothing is impossible. But, you are likely ok. However, the risk with the 5 volt is more likely that it just won't power up and function at all, whereas the risk with the 7 volt is more likely that it could blow something (damage the electronics). There is also a current rating, but it seems likely that the monitor would not have a huge requirement for current. IANABME (I am not a baby monitor engineer). So, if getting a 6 volt source is not an option, I would try the 5 volt first, and if it doesn't work then try the 7 volt, with the knowledge that there is a small chance it could break the monitor. But (in the USA anyway) anything made for 6 volts would normally be able to handle 7 volts. Normally.