No, this is not a request for software!

My two kids bedrooms are in the roof of the house and have velux
windows, but because all the sun in the morning falls on the front of
the house, my daughter's room becomes like a sauna any time we have
sunny weather.

I have seen that people make solar reflective film to go on the insides
of window to cut down on this problem.

What does the team think of this idea?
Is it effective/too expensive/difficult to get hold of?

Any advice gratefully received..

dedics