I am closing soon on a duplex in St. Paul, MN which I intent to owner-occupy. There is already a month to month tenant in the other unit with 2 people listed on the previous lease. It’s a 2 bedroom unit and there are plenty of toys around so I assume at least one minor also lives there (though I have never seen any of the occupants). I’m going to present the tenants with a new lease and would like to make sure all persons over the age of 18 are on it (and would also like to know how many total people are living there). Is it against Fair Housing laws to ask the current occupants to fill out the names and birthdays of all the people currently living there? Or is there a better and legal way to approach this? Thank you in advance for your input. 

You can ask number of occupants at any point before or after occupancy. I usually dont ask for birthdates but rather for month/year of birth or age on move in. ( birthdates are considered an unnecessary privacy risk in my day job.). Age is needed for lead laws if you have an older building.

What would you do with the info if you knew they had or did not have kids?  

Are you trying to lock them in on a longer lease or do you just want them on your own lease forms?

I don't think you can ask prospective tenants if they have children, but if they are living there and do have children, I'm not sure why you care to get it listed on the lease unless your plan would be to kick them out if they have children in order to find other tenants that do not.

I am not an expert, but wouldn't think it would be a problem if you require everyone over 21 to be on the lease.  

If you plan on living in the other half of the duplex, I'm guessing you will figure out who is living on the other half as it is.  

What ever you decide make sure you keep them on a M2M lease to protect yourself. They may or may not work out and for that reason you do not want to place them in control of your property with a term lease.