Why bother with families at all? Just introduce a market-based wage for having a kid, paid till the age of 18, inflation adjusted, but set at new every year based on supply and demand (say decide you need TFR of 2.1 and that's it). Poor women will quickly fulfill the demand. They are not at all fussy about kids. It won't be too expensive. A woman can have 6-8 kids biologically, if half of the 20% most poorest will do it, it will fill the gap between real and desired TFR.