WHY WONT MY MII's HAVE BABIES

At least one of the Miis relationship status with their spouse has to be on soulmate. You can check by clicking on the left or the right button next to the game slot

I am sorry for being a year late, but just in case you still need an answer: They don't need a mobile phone to have a baby, they have to want a baby. One of the married miis will have the heart icon in their window of their house/apartment and if you check, they should say something like "(Mii) and I have been thinking about having a baby..." And you will have the option to say If you thinks that's ok or not. But you should get a mobile phone so that when they have a baby they will call you. Also, they need to both be either light or dark green in their relationship. Yellow/Orange is okay, but they're less likely to want kids. If they are in red, lavender, or purple, they won't have kids, and might break up. You can check the relationships by pressing R when visiting a mii while in their apartment. Also, check if you have the maximum amount of islanders. If you have 100 islanders, you can't have kids because they won't have space to move into the island when they grow up.

Im 2 or 3 years late but the miis relationship have to be status Soulmate. And if you go to town hall click the button on the bottom right corner, click it and it should be a button with a baby on it then click it then it should say allow miis to have babies and then click yes then go play tomodachi life for a while. If two miis don't fight a lot and both own cellphones, one day one mii will have a heart on their window and ask to have a baby [hearts don't always mean a baby. And to have a baby, you need a married couple] hope I helped!