We have blackout lined curtains, which don't let light through, but it comes round the edges. For that reason we also have the Gro Anywhere blackout blind that suckers to the window pains. This is excellent, and doesn't let in any light, but it also means we can't open the window.
DDs room is south-facing, and it was 24 degrees in there the other day, and it's only April. I'm afraid it's going to reach about 30 degrees in the summer if we can't open the window for ventilation! We tried a night without the Gro blind last night and she woke up at 4am and wouldn't go back to sleep.
How old is she? Is it possible that after a few days without the blackout blinds she's get used to sleeping in the light? Especially as she's south facing ago the sun shouldn't be on the window first thing (sun is now rising to the north of east).
On a more practical and helpful note, we used something like this. If you cut it out for each window pane, then you can still open a window if needed. Obviously light will come through the one open window crack, but at a much reduced level to having no blackout at all. If you want fresh air, there has to be some kind of light.
Just be warned, the sticky stuff is a bugger to get off if you leave it on for years months. Although hopefully the product has changed/improved since we used it.
brainonastick you're right, it hadn't started to get light yet at 4am. I think the problem was that there's a streetlight right outside her window, so the room's quite bright even when it's actually dark outside.
I've been considering the stuff that you suggested, but some of the reviews say that it doesn't stick very well and falls off!
Get rid of the gro blind and get a blackout roller blind to go behind your curtain. That's what we have and you can open the window. It still gets hot in summer so they sleep in light clothing and - crucially - door is kept open to allow air to circulate and breeze to pass through, otherwise it's unbearable stuffy.
When it's breezy we sit a heavy (bean bag type) teddy on the window sill leaning against the blind and it stops it from rattling - we have the same combination in our room as DH is as light sensitive as a baby