What WMMC said. You'll need to talk to a glazier - in our case they replaced the entire pane of glass with one with a circle (you'll need the catflap dimensions) cut out of it, but that might not be necessary.
Cats and children are fine together. You'll need to discourage tail pulling. If you're getting a kitten they can't go outside for a quite a while anyway - not until they've been neutered.
There are catflaps for glass doors, I had one once. Is it double-glazed? If so, you might have a problem, as double glazing involves some kind of vacuum between the two layers of glass and if you cut it you will be releasing the vacuum - not a good idea.
When I used one for glass doors, I discovered it wasn't actually glass but perspex. I had borrowed one of those electric saw things with a very little blade, which basically melted the perspex! I had to do it in very short blasts so the bloody things didn't heat up too much. Got in done eventually though, and for all I know it's still there!
Don't know about the cat flap but a cat makes a great pet for small kids. They are just the easiest pet. No walks, no cleaning out hutches or tanks so easier than a goldfish imo.
I have 2 cats at any one time - and older one then get a kitten when the elderly one dies. So at the moment I have an 8yo cat and a 4 month old kitten who is sooooooo cute and friendly.
If you get one from cat's protection they will make sure that it's a kitten that has been well socialised so will be friendly with your children. They ask for £80 but that includes microchipping and the first year's immunisations.
Putting them into walls is pretty easy actually - a lot easier than putting them into glass doors - we had one which was two foot up and we put a little shelf under it and the cats used to love it! Was easier to clean as well because you didn't have to bend down much.
We have a cat flap in a glass door. When it was installed, the glazier replaced the whole glass pane with safety glass as he said the strength of regular glass is compromised when you cut a hole in it.
We're waiting to hear back from the cats protection about getting a cat, had our home check last week . We've not got a cat flap and won't be getting one put in as there is nowhere to put it. There is always someone in the house so they will be able to let the cat in/out as it needs, that's what I'm hoping anyway.
We only have to pay costs of £30 for our cat, but then I don't think they do microchipping I think we need to do that ourselves. Must remember to ask them when they phone.