They were a tyranny in our house too! I bought a Sodastream, HobbyBrew CO₂ cannister and an adapter to join the two together. For the cost of like 30 bottles of Diet Coke I now have cheap fizzy water and the kids can add squash to it.
I looked at doing this but calculated that the CO2 bottles work out more expensive than just buying bottled sparkling water from Lidl.
Yeah the CO2 bottles and exchange is expensive as hell. It's apparently cost effective if you use a commercial one but the outlay is pretty high. Not worth it either way IMHO. I just put up with flat shit.
There's also Sparkel, which uses a baking soda + citric acid powder in a separate reaction chamber. The cost is pretty low per bottle if you buy the bulk powders and mix it yourself; has been a while but when I had a Sparkel, I think I worked out that the cost was somewhere under 10 cents per 750ml bottle.