More light, and take away the association with the sun. Basically, try to get them to respond to light when you want the day to be.
About an hour before the sun sets, close any curtains etc and turn all the lights in the house on (and, if possible, TVs and extra lights). This will increase the "perception" of daytime, and remove the "Oh look, the sun's setting" trigger
Allow the child to look outside if they wish to see that the sun is still up, but aim to distract them with games, toys, food etc: anything that takes their mind off what's happening outside. Be active yourself, too.
Then as it gets closer to the bedtime you'd like, start reducing light levels and noise, so you're settling them down in much the same way they did to themselves.
Keep their room as dark and quiet as possible in the morning until about half an hour before you'd like them to be up, then start increasing light levels.