Your child is born after you get married in Harvest Moon DS: Sunshine Islands. The next step is to make sure your house is the largest size possible. You actually can't upgrade your house from the marriage-required large size until after you are married. Talk to Gannon when you are ready to upgrade your house to super-size, which requires 700 Wood Lumber and 50,000 G. Alternately, you can pay him a flat rate 90,000 G if you don't have enough lumber on hand.