I have a sodastream and use it daily, and I've read articles going both ways on this. It seems that carbonated pure water is not nearly as bad as soda for teeth, but that perhaps it's not that great (especially for kids). Have you seen definitive guidance on this? Does carbonating less make a difference? Using a straw? I've heard that drinking plain water after is a good idea, but don't know if that's grounded in science.
A quick google shows that plain carbonated water has a pH of 4, which is right around where it becomes corrosive to enamel. So, if you just drink a bit of water afterwards, you should be bringing your mouth's pH well above 4. Maybe swish it around as well.