How did you fix your boiling hot house?

[Octonaut4Life]

The upstairs rooms at the back of our house are boiling, boiling hot in the summer, which is a bit of an issue as our toddler now sleeps in one. Neither have excessively big windows and we have black out blinds up on them to keep the light out but they get so hot and they cool down incredibly slowly even once it's dark and significantly cooler outside. We're in 1920's ish mid terrace house.

Have thought about trying to put air conditioning in but I don't think the toddler's room would be suitable, because there's barely any external wall you could have the outside part on. Plus it's expensive and I don't really understand why the rooms get so hot in the first place.

I don't think the attic is particularly well insulated so have wondered whether improving the insulation up there would help. But really looking for any tips/thoughts and anyone who has a similarly hot upstairs who has managed to fix it!

Last summer in the worst of the heat, I put white dustsheets on the outside of my 2 south-facing windows. The top half of the window opens horizontally, so I shut the top of the dustsheet out of the top of the window, and then carefully closed it (sorry, that explanation is a bit rubbish). It's a very temporary solution, and looks ugly from outside, but it kept the rooms siginificantly cooler than just closing the curtains.

Some years ago I tried taping white paper to the inside of the double glazing, to reflect the heat. It sort of worked, but both panes got blown, so I don't recommend it.

Has anyone had any external window shutters fitted? Any company recommendations?