Remote Mac sleeps when closing ARD

I use ARD on my office Mac to control the iMac we have set up in the kitchen for our son. I can take control with no problem, but an issue arises when I have to log in while the iMac is sleeping. It wakes up when I connect via ARD, and I can watch what he's doing, but when I close the ARD window, the iMac immediately locks and goes to a black screen. When I reconnect and log in again the same thing happens. We don't have this issue when I take control while the iMac is already logged in--in those cases I can take control and close ARD without it affecting the iMac--but if I have to log in, I then need to leave my ARD window open on my Mac, otherwise the iMac immediately locks. Is there a way to stop this from happening?