There's an "Okay to Wake" clock which may be easier for a 3.5yr old. We also had to use a clock to tell DS not to wake us before a certain time. We didn't say he couldn't come out of his room because he's very literal and wouldn't even go potty!
https://www.amazon.com/OK-Wake-Alarm-Clock-Night-Light/dp/B00EAHSBV4