In addition to the financial reward it already offers for the birth of new babies, what other steps could the government of singapore take to guarantee its future financial security?

Fund and improve the standard of schooling, this will mean more children will gain the qualifications they need to work and therefore pay tax. It also eliminates the need for money to be given out to support people who are out of work.
Make it easier for skilled, educated migrants to work in the country, they increase the working population so more people will be paying taxes.
Give tax benefits / priority housing, healthcare, schooling / reduced public transport pares / assistance with paying for childcare or other economic benefits to families with a certain number of children (in France for example this is 3 children per couple)