1. If you're going to fly with kids under 2 years of age (“U2”), the U2 kid(s) should preferably be in carseats, not on laps.
2. If the cost of buying seats for your U2 kids to fly comes with engaging in riskier behavior (such as driving) or foregoing payment for more sensible risk mitigation measures (than paying for a seat for a U2 kid), then put the kid on your lap and fly.
In a world of limited resources and far greater risk for children than traveling as a lap-child on a plane, the money used to buy the U2 child a seat is generally better spent in other ways to maximize the well-being of children.