I'm adopting Sophie (Her parents ((Father?)) were ((was?)) a Stormcloak and I want to take care of the child of a fallen brother and sister ((Just brother?)) of Skyrim) and Hroar (I want at least one male child, want a male heir to take my place in the Stormcloaks), but I can pick a wife between Ysolda (Spelling?) and Mjoll. Ysolda wants to be a travling merchant like the Khajiit, Heljarchen Hall being a great location. On the other hand Mjoll is an adventurer (Ex adventurer?) and could get killed, plus I dont see her combat skills translating to motherly skills.
Mjoll is labeled an essential NPC and can't die--EVER. That's why she's one of the most popular choices for follower/adventurer/spouse.
WARNING:
I married Mjoll, but marriage to her seems really glitchy. I have both Skyrim and Hearthfire updates. When I built HH in a test game, I already owned Honeyside and Proudspire (both fully upgraded & with child bedroom). If you marry her Arin won't move in because the Skyrim update fixes the stalking glitch. You'll get the option to move her to the pre Hearthfire homes. But as for the DLC ones, be aware the option for these homes may not come up in her dialog. Also Greggor may not show up as your housecarl at HH. The dialog doesn't come up in the Jarl or his steward's conversation. So I tried to ask Greggor directly. But this NPC apparently dissappeared from Dawnstar.
So I was forced to use the placeatme cheat to summon Greggor from whatever plane in Oblivion he was hiding out in. Talked to him and immediately got the option to make him my steward. He showed up fine after I fast traveled back to HH. I also had no issues hiring Sven as steward and had him hire the bard, carriage driver, livestock etc.
The only NPCs missing at HH were the kids and spouse. When I tried to move in Sophie and Lucia into HH, only the vanilla options for Solitude and Riften came up.

So I moved them into Solitude instead and then fast traveled to Riften to marry Mjoll the same day. Asked her to relocate to Solitude, then fast traveled back from Riften. She was there when I got to Proudspire with the 2 girls. But when I tried asking her to relocate to HH, the option didn't come up.
I tested a very early save where I only owned Honeyside and rebuilt HH with no changes in my mods. I removed all workbenches in the fully upgraded rooms. Apparently, the game thinks the house is still being built if you leave these in and may cause glitches.
This time around, I married Mjoll first and then adopted the 2 same girls afterwards. The same bug happened-couldn't move her into the HH, only into Riften.
So I threw caution to the winds and added in the Spouse can live anywhere mod--expecting it to crash the game on loading which it never did--and reloaded the same earlier save game. Except this time, I activated the mod to mark HH as a valid home. Then I went to Riften and married the Mjoll. Right after marrying her in Riften, I asked her to move in to HH and the option came up in her dialog. But not trusting the game, I had her follow me back to the house then I dismissed her as a follower. She immediately said she was returning to Riften, but I was able to ask her to stay at HH in her dialog. So she ended up going into the upstairs bedroom instead.
Then I fast traveled to get Sophie and Lucia and had them move into HH. Mjoll was still there on both occassions along with the bard and all the other NPCs.
Note: On all occassions, Greggor the HH housecarl failed to materialize at the house. I tried talking to the Jarl and his steward in the test game where I made Mjoll a follower, but no option for this came up. In fact, it appeared Greggor had completely dissapeared from Dawnstar?

Regardless, I was able to overcome the bugs by using the console to force Greggor to move to HH, and the Spouses mod to force Mjoll to stay at HH once I dismissed her as a follower. So far, I've only had to use the console placeatme cheat only once to force her to reappear in HH.

But so far, she hasn't wandered off. She usually remains inside the house along with the bard. Sven and the kids will frequently go n and out of the house to wander, do chores/play around the house. And Greggor is typically outside patrolling the house perimeter.
Also on all 3 occassions, I noticed my Nord wasn't autonomously equipped wearing his matrimonial ring? I also noticed Mjoll also didn't have her ring equipped either? when I made her my follower in the last test save. I was exchanging some equipment with her as my follower, right after marrying her in Riften when I noticed it. So perhaps this bug may be associated with her autonomously not wearing the wedding band?
So if you marry Mjoll and not using mods, you might want to make her your follower right after marrying her. This way you can verify she's wearing her band (and your character wearing theirs as well). Then have her follow you back to the Hearthfire homestead. I'm not sure if Mjoll will attempt to wander off back to Riften if you dismiss her once you get to the new dlc home however.
It's really ironic I have to use the Spouse live anywhere mod just to move her into HH though.

I'm not sure if this is the case with the other 2 homes. Hmmmm.......