For this question I'm assuming that for the first few Island One's
and perhaps even into the Island Two series, that transport of a
colonist is expensive so that there is a natural incentive towards
high birth rate in the Habitat to try to lower the demand for
immigrants.

From a public policy POV, how best to achieve that result.

How would you feel if your social security payroll tax was lowered
for every child you had? Afterall, the program is a pay as you go
system and the number of workers supporting a retiree is growing
very small, so if in the habitat you contribute more children then
you become less of a burden to future generations. Let's assume that
the reduction in taxes was meaningful.

Further, because the Habitats will be so small in area, with
everything pretty much within walking distance, daycare at work is
commonplace, one hour commutes are unheard of, the parents don't
have to escort their children to school to prevent the perverts from
kidnapping them, the children can go play outside unsupervised, and
other things that would make the society very child friendly. Would
the environment and the tax policy act as an incentive to have more
children?

TangoMan

For this question I'm assuming that for the first few Island One's and perhaps even into the Island Two series, that transport of a colonist is expensive so that there is a natural incentive towards high birth rate in the Habitat to try to lower the demand for immigrants. I think a prerequisite for building these things is cheap flights.

I've been studying this, and flights as cheap as about $2000 per person in todays money is theoretically possible; that's the asymptotic price of the fuel anyway. It seems reasonable that the cost will eventually reach this price, if nothing better comes along.