After marriage and your spouse moves into house, and you have upgraded it to 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 house near the bed.
Wait 30 days from the date of your wedding. Since the seasonal calendar has been removed, players must to remember or write a note to themselves when their wedding date was. In the morning the female side will not feel well. Jeanne will be summoned to the house, where after a quick checking-up she will announced that the two of them are expecting a child.
The next event happens 30 days from the pregnancy announcement. In the morning, your spouse will ask about the ideal gender for our child. The player can pick a gender or have this game decide one at random.
Lastly, you must 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. You will also receive a Rattle, Picture Book, and 10 Baby Formula. Rowan will appear and let you know that the sprites will watch will watch over your child while out of 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 one. The recipe will be added to your kitchen if you want to make more formula.
- Formula Milk = 2 Milk or 1 of higher quality milk
- Creamy Formula Milk = 2 Chocolate Milk or 1 of higher-quality chocolate milk
Bastian will also sell Baby Formula at his restaurant for 5 G each.
After 30 days you will awake in the morning to find your child crawling around on the ground. The child will start to walk after another 30 days.
After your child has walked for 30 days (basically 90 days since birth), one will wake up in the morning to see him/her in bed and feel very unwell. Jeanne is called to your house to take a look, but even she isn't sure what is going on.
Head over to Edmond's house to seek his assistance. Edmond has a bad feeling about this...
Back at the house, Edmond concludes that your child 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 child 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 child's body. He needs 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 will 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 will also need a potion to dissolve the light and directs one to go talk to Harvest Goddess.
The goddess confirms that you must need that specific potion. Edmond knows how to make it. She reveals that you will need more than just the gem and the potion; you will also need a special Magic Pot to mix the ingredients together. Harvest Goddess doesn't know how to obtain a Magic Pot and suggests you go talk to the "weird guy".
Visit Doc inside of his laboratory. Doc confirms he can craft a Magic Pot if you bring him 10 Hardwood Lumber, 40 Glass Material (the original ore; not processed Glass), and 1 Soul Gem. Once you have the materials, go back to Doc's lab and he will 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 house to administer the cure to your child. 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 will find your fully-grown kid walking around the house.
Source: https://fogu.com/hm/light_of_hope/marriage_kid.php, with some changes.