British Columbia

Hmm... As to the second point, I'd argue that people with dependants are contributing, by having dependants. Giving them a tax break is sort of like paying them to take care of things that the state would have to take care of otherwise, in the form of orphanages, daycares, food banks, public nursing homes, etc. At that point, it's just an efficiency question: is it better to tax parents less (so they have to work fewer hours and can take care of their kids), or is it better to run more after-school programs (so the parents can work while someone else takes care of their kids)? Should we tax them less so they can buy food and shelter, or just give them food and shelter? The answer isn't cleanly one or the other, but falls somewhere between "give them money (by taxing less)" and "give them stuff" for each thing that people provide for their dependants.

As for overpopulation, once people are already born, it's too late. Incentives should prevent people from being born in the first place, but not punish the parents of the already-born (and the already-born themselves). To do that you could do normal birth-rate-reducing things like comprehensive sex ed and ensuring easy access to birth control, or go at it from the other side: streamline the adoption process and incentivize people to adopt rather than procreate.