Always mount blackout blinds inside the window frame, not over it. It's surprising how much light can bleed through the narrow gap in the sides. If you do this, you won't need curtains to catch the extra light.
If for a kids room and you want to do for cheap, I used these.
https://www.ikea.com/ca/en/p/schotti...gray-90369507/
Cut them to size and mount inside the window frame. The tape on it sucks though so you either should use a thumb tack to hold it up if a wood frame, or a stronger double sided tape of a plastic frame.
Worked fine for me and room was usually too dark to notice that it looked cheap.