Yeah, I have been wondering about that, too. I've found these potential reasons:

1. Self-selection bias. If you're optimistic enough to have kids, you're probably more optimistic about the climate in general.

2. Genuinely needing things. Not an SUV, but e.g. grocery shopping without a car, when you've got a family, is quite a challenge.

3. Other things to worry about. Parents are often just trying to survive the every-day-chaos. Worrying how to save the world has a lower priority to them.

4. Someone to be selfish for. As an individual, I'm free to be altruistic, because it only affects me. But with a partner and/or kids, you can always buy nice things for them instead.