My daughters ds lite power is on it's a green light

I had that issue also.
I opened the DS, and I saw that a connector was not attached to the mainboard. I connected it, and the problem was solved.

Be careful opening the DS.

Sounds like the processor has gone on the mainboard.
Using r4 or similar card? if so these burn out the board sometimes.

well go online type get ds fixed the they will tke it for 2 days

rerewrewewewrerw

there isnt a reset button. have you any power lights or screen flashes.

Problem ia LCD Screen or systemboard. Are both screens dark? If so then it is the systemboard

When you boot the Nintendo DS you can change the illumination settings in the main menu.
One the main menu, you will find a sunny symbol in the lower left corner of the screen. Click on that and you should be able to change the level of illumination. If you are also wondering if you can change the brightness during game play, you can't. Games such as Mario 64 DS come with support for brightness change but otherwise you have to do it before game play.

Ditto... except my daughter is 10. Both screens remain black, are fully charged and green light stays on for more than 4 minutes.

before i answer how many times had you charged since you bought the ds

It's not likely to be battery, but fracture in ribbon cable to top screen causing this. Both screens must be connected or you get the symptom you've explained.