Do I need two nanit plans for two children?

If you purchase a second camera it will come with 12 months basic insights included. You can use both cameras on the one account (and use the Split Screen feature. More info -  https://www.nanit.com/blogs/parent-confidently/introducing-split-screen) 

Without an active Nanit Insights Plan you'll be able to access camera, audio, room temperature and humidity, motion and sound detection, two way audio, white noise, night light and access live stream. You can also take still shots that save to your camera roll on your phone. 

However you won't be able to access breathing motion monitoring, milestones tracking, sleep info, and other insights. 

So you would need two active plans to receive insights that are unique to each child, specific to their age, breathing and sleeping patterns.