It runs multiple ways - yes, children used to be the ideal pension plan. When childhood diseases were often lethal, “heir and spare” was a better strategy. Also, children could help around the farm or shop. Even a 6 year old can watch the goats and keep them corralled (usually). Isaac Asimov recounts in his autobiography working in his father’s candy shop at a young age. etc. Rags-to-riches or hard-luck stories are full of teenagers who dropped out of high school to help keep the family afloat. Heck, even in the 60’s and 70’s, I worked for my pocket money delivering newspapers and as a caddy, from grade 8 on.
Today, far less of these opportunities exist. Indeed, children are a much larger drain on resources. Nobody cared about brand names when I was a child - jeans and sneakers were just that, nothing special. Toys were pretty cheap, since none had chips in them. “play” meant go outside. The cost of an extra seat for each child to fly somewhere on vacation was irrelevant when far fewer people flew places. My parents left us to our own devices for the afternoon or evening far younger than society or the law seems to allow now.
Then there’s the inconvenience. My parents didn’t go too far out of their way for us. If it’s a mile or two to the library or swimming pool - take your bike. Parents nowadays seem to spend their after-hours shuttling their children from one activity to another. I walked to school or took transit. Today, the road outside th local school is plugged solid with parents waiting to pick up junior for the 4 blocks drive home.
And the mantra “I’m why they can’t have nice things”. Not just vacations, but so much else in life costs money, and money spent on children subtracts from things like the latest iPhone or that 75" TV.
I would guess that the problem with declining birth rates has as much to do with the escalating cost of children, coupled with inconvenience of raising children, the escalating costs of life in general. Perhaps anohter aspect of human evolution is that with birth control and economic disincentives, we are selecting future generations for people who instinctively really want children over other lifestyle motivations.