After you're married and your spouse moves into your farm house, and you have upgraded to the Large House, Doc will have a Crib available in his insta-building shop. Building the Crib requires 2,500 G, 5 Hardwood Lumber, 1 Silver, and sheep wool; either 5 normal Wool, 4 Great Wool, 3 Ultimate Wool, 2 Supreme Wool, or 1 Miracle Wool. After supplying the materials to Doc, the Crib will appear inside your farmhouse near your own farm bed.
The next step is to wait 30 days from the date of your wedding. Since there's no calendar in the game, you'll need to remember or write a note to yourself when your wedding date was. In the morning the female side of the couple will not feel well. Jeanne will be summoned to the farm house, where after a quick checking-up she'll announced that the two of you are expecting a child.
The next event happens 30 days from the pregnancy announcement. In the morning your spouse will ask about your ideal gender for your child. You can select that you want to have a girl, have a boy, or that you don't mind either way; selecting the third option will allow the game to select the gender for you. This secret gender won't be revealed until the birth.
Lastly, you've to wait an additional 30 days after the gender selection. In the morning Jeanne and Carol will come by to help with the birth. The baby will be the gender you selected 30 days prior or randomly a boy or a girl if you selected that you didn't care. You will also receive a Rattle, a Picture Book, and 10 Baby Formula. Rowan will appear and let you know that the sprites will watch will watch over your child while you're out of the house.
The new little baby will be placed in the Crib. You can feed your child every day using the Formula that Carol gave to you. The recipe will be added to your kitchen if you want to make more formula.
Bastian will also sell Baby Formula at his restaurant for 5 G each.
After 30 days you'll awake in the morning to find your kid crawling around on the ground. The kid will start to walk after another 30 days.
After your little guy has walked for 30 days (basically 90 days since birth), you'll wake up in the morning to see your kid in bed and very unwell. Jeanne is called to your house to take a look, but even she isn't sure what is going on. Perhaps Edmond might have an idea?
Head over to Edmond's house to seek his assistance. Edmond has a bad feeling about this...
Back at the farm house, Edmond concludes that your kid has been infected with Petrification disease. This plague infected the island a long time ago. If left uncured, the victim will eventually turn into a rock! Your kid has only recently become infected, so there is still hope for recovery. The Harvest Goddess and Harvest God may have a few remedies to counter the disease. Edmond instructs you to go visit the Harvest God while he stays behind to prevent the disease from becoming an island-wide outbreak.
The Harvest God explains that his power can cure Petrification, but it must be turned into a remedy that can be consumed, so its light can cure from the inside of your kid's body. He'll need a gem, but has forgotten what it was called. Gorgan might know though!
Gorgan says curing your child has nothing to do with him, but you guilt him into cooperating. The Underworld King says you'll need a Soul Gem to condense the light. You can find the unprocessed ore in the mine from floor 71 to 100; just go down and smash rocks until you locate Soul Gem Ore, then take it to Gus' blacksmith shop to process it into a Soul Gem. Gorgan explains you'll also need a potion to dissolve the light and directs you to go talk to the Harvest Goddess.
The goddess confirms that you'll need that specific potion. Edmond knows how to make it. She reveals that you'll need more than just the gem and the potion; you'll also need a special Magic Pot to mix the ingredients together. HG doesn't know how to obtain a Magic Pot and suggests you go talk to the "weird guy."
Visit Doc inside of his laboratory. He confirms he can craft a Magic Pot if you bring him 10 Hardwood Lumber, 40 Glass Material (the raw ore, not processed Glass), and 1 Soul Gem. Once you have the materials, go back to Doc's lab and he'll make a Magic Pot for you. As you start to leave, Edmond catches up to you and hands over the potion that will be used to dissolve the light.
With the Magic Pot, Edmond's potion, and 1 (additional) Soul Gem in your bag inventory, return to the Harvest God to create the potion. He'll say a few magic words and then hand over the Petrification remedy.
Finally, head back to your farmhouse to administer the cure to your kiddo. The remedy appears to be working, but it is too soon to tell if it is a complete cure. You automatically go to bed for the day.
The next morning you'll find your full-grown kid walking around the house.