Can you get married if you already have a child

You can let him/her get profits, cook food and get a 15% skill-learning boost after sleeping together.

After the Marriage, your partner will ask where do you want to live and you will have the choice of living with them, or if you have a house, live there. If your spouse relocates to a different City from where they originated to live in one of your houses, they may indicate that they wish to become a shopowner to keep occupied while you are adventuring. If this occurs, you can ask them once a day how the shop is going to receive a share of the profits (approximately 100 gold per 24 hours). You will also be able to buy and sell from/to your spouse as would a regular merchant NPC.

Once a day, you can ask a spouse to cook something for you. They will make a Homemade Meal that increases the regeneration of magicka, health, and stamina. If you sleep in the house with your spouse, you will get a temporary bonus called, 'Lover's Comfort' that boosts your rate of skill-learning by 15% for eight hours.

— Wikia - Elder Scrolls V: Skyrim - Marriage

If any, watch out for the following two bugs:

• A bug can occur during the wedding ceremony at the Temple of Mara. After Maramal pronounces the player and the player's NPC of choice married, the NPC may start walking toward the temple's exit before Maramal is done speaking and player can move. If the NPC leaves the temple before the player can speak to him/her about where they will live, then the NPC will disappear. They will reappear if they are part of a quest, like Ysolda in "A Night to Remember". However, after you say "I do" if you exit the speech and click on your spouse fast enough before he/she walks out the door, he/she will not disappear. But even if you tell them to move into your house they may or may not actually be there. They will be at their prevous house instead.

• A bug can occur in which a scripted NPC interrupts the wedding if you choose to wait overnight in the Temple of Mara. For example: the priestess of Azura might make you miss your wedding if you recently complete the Azura's Star quest. What results is that you miss your wedding, but still have the option to make it up to the other person.

— Wikia - Elder Scrolls V: Skyrim - Marriage

Yes, for you it'd mostly be for role playing purposes. There is one exploit that may or may not be beneficial to you depending on how much you need money. As that 100 septims stack, you can sleep for 24 hours 10 times for example and get a 1000 gold. This was slightly helpful for me when I was trying to buy loads of Master spells at 3000 each.

Apart from that its pretty much useless gaming wise especially if you do not need money.