I would have the windows sealed until the children are older.

To be safest, I would have them sealed and only open the top windows. It’s better to be safe then sorry I think. Have them unsealed later on maybe

I would ask for advise from various emergency services. They should be able to advise you of something that will keep your children safe but make it suitable for you to be able to escape from in a sheer emergency. You don’t want a criminal to break in but you may need emergency services to be able to – I hope not but if you are prepared there is safety and rescue plans available that way. If you opt for key locks put them high enough that little ones can’t access them but that you can get to quickly. If all the locks are the same make sure they supply you with as many keys as there are locks. You probably wouldn’t have time to run from window to window to open it with one key.
I know of a new unit which has two sliding windows with matching locks and they only supplied one key.

Security bars on windows are only good at stopping thieves entering though windows BUT THEY ARE A DEATH SENTENCE FOR HOUSE OCCUPANTS IF A FIRE SHOULD OCCUR. You really need to be able to escape through a window if you have to or have a fire fighter climb though one if necessary. Locks that allow the window to be open a tiny amount are good at preventing children from falling out the window and in the event of a fire at least the window can be smashed. But I also recommend talking to the fire brigade/station.

Never EVER seal windows or entryways, as this is a fire hazard! Get a locksmith to fit key locks onto the windows so you can open them enough for fresh air, but not wide enough for a child to climb out.

can you get security screens??

A simple nail on both sides of the bottom pane will do the trick. Cheap as chips and no handyman needed

I would be putting up some nice but solid security grills on the windows and pick a style with a pattern not just the plain up and down bars. You could put a window lock on the window…if they are slidy ones get the ones you can lock and if it is the old push out windows get a bolt to the top of the windows

Whatever you decide. Be it safety locks or security screens you also need to bear in mind how they can be accessed in the event of a fire. Try talking to your local Fire Department or local Children’s Hospital usually having a department that helps with safety issues.

Hard to give you a definitive answer without knowing the style of your windows, but for sliding windows you can definitely buy safety gadgets/ window locks which will stop them opening more than a centimetre or two. You may also want to consider security screens for the windows – to stop the kids falling out, not people getting in.

I’d go with security locks….

I would definitely get locks which I sure places like Bunnings would sell them & get a handyman to install.

If you head to any baby store you will be able to buy window locks or you can call you local window guys and get them to put key locks on the windows to.

I would go for the Handyman solution!!

I’m sure you can have something fitted that only allows them to be opened slightly. You will need some fresh air flowing in. Heat rises. It can get awfully stuffy ant hot on the top floor if a double story, so I’d imagine a 3rd floor would be worse.