You give incentives to have children.
Simplistic answer, doesn't fix the problem now, and takes at least 18 years to produce a tax-paying worker. What are these incentives and more importantly, how do you fund these incentives? If you don't know, then what makes you think it can be done? Which is probably why the government went for the immigrant option, which definitely does work in terms of boosting the economy and generating tax revenue.
Raising a child to adulthood in increasingly urbanised societies, and helping them buy their own home is a huge cost - far more than any government could afford in terms of incentives. Having a child is a huge open-ended responsibility as there is no guarantee that they will become a productive worker, will have the skills to get a job or will even want to do the jobs where there is a shortage, and will not have any health issues that prevent them from working.
Also parenting usually results in the mother's earnings being reduced from what it probably would have been if she had not had the child, since she would either have to take time away from her job to care for the child either by reducing her hours or staying at home for a few years. So the government would have to provide enough money to compensate for lost earnings, a bigger house, nursery, education, clothing, food, not to mention the risk that she might end up a single parent etc
People are more likely to have more than one or 2 kids if they sense there is an expanding economy in which they think their children have a chance of a better life and there is good family/ community support to help care for and nurture the child to become a productive adult. Potential parents are usually motivated by access to affordable childcare and good education, access to affordable housing, cheap mortgages. This is not a description of the economy we currently live in, whereas immigrants do boost the economy now - immigrants mean more people working, spending money in the economy, paying taxes, providing resources to pay for the ageing population.