Full House

Marriage first: 10 hearts with a bachelor/bachelorette (bouquet at 8), house upgraded at least once, then the Mermaid's Pendant from the Old Mariner - he stands on the tide-pool beach past the bridge on rainy days, 5,000g. Wedding is three days later.

Kids next: with the nursery upgrade (Living Large's second tier), your spouse will eventually ask if you want children - or you can adopt; both count. Two kids, each a couple of in-game weeks apart through the baby stages, and it pops. Same-sex marriages adopt automatically. Patience is the only ingredient.