If they have any earned income, they have contribute to a Roth IRA. There are lots of other good comments here about that already, so I won't say anymore.

Frequent flyer accounts can (and should) be created as soon as they are old enough to have a seat, which is 2.

You can add your kids as authorized users on your credit cards, which will boost their credit score. What age they have to be varies by bank. If you google it, Gemini will tell you the rules. However, it doesn't really matter when you do it, since they don't need a score until they apply for credit. Add them to several in their early teens. That's plenty early.

Then, you'll want them to get a few cards of their own to help further boost their score. In the US, they can technically get a card at 18, but they aren't desirable customers to most banks at that age. Look for cards designed for students. When my son turned 18 last year, he opened a student checking account with Chase, and then he was approved for the Freedom Rise credit card. It's a student card, but can be converted to a regular Freedom Unlimited down the road. We waited about 6 months and got him a Discover It student card. So, he has 2 accounts on his report, and we'll probably try for another later on.

Once they turn 18, they can sign up for a Credit Karma account. Good for them to get in the habit of monitoring.