Why not just introduce a competitive, market-based system and let the market decide? Give a monthly subsidy to every woman for having a child until the age of 1

Why not just introduce a competitive, market-based system and let the market decide? Give a monthly subsidy to every woman for having a child until the age of 18, regardless of marital status and number of children, and fund it by a flat per-head tax (flat as in, in $, not %). Adjust amount of subsidy as the birthrate changes to keep it at the optimum level, locking it in (inflation adjusted) till kid reaches 18. Poor women will be compelled to have kids because they can't afford the tax. Because they can have many of them, easily 5-6, and it will immediately become competitive as subsidies fall as the market is saturated (we all know too well how any market with no entry barriers and guaranteed pay works - it is immediately inundated by supply, see Upwork) - the resulting fee will be very low. People who refuse paying tax claiming no income - send them to work (women) or military service (men). In a poorer country that could result in mass emigration, but Norway is rich and no one in a sane mind will escape it. Problem solved.