How do I address my tenent having a live-in guest?

[Brad Crosby]

I began renting out my 3 bed 1 bath in November to two tenants that are roommates living in separate rooms. Less than a month later I stopped in for a scheduled maintenance request and found one tenant's boyfriend hanging out. I didn't think anything of it except that I saw they had put a mattress in the 3rd bedroom, which I thought was odd but not necessarily condemning since there is a built-in box in that room so there isn't much else to do with the space. When I returned to the house a couple weeks later to repair a door upstairs, the boyfriend was there again with the tenant, and he had his two kids there as well. As I was working on the door, I noticed the mattress in the 3rd bedroom had been removed, but the room was full of things as if it was being used for a bedroom (guitar on a stand, books up on shelves, etc.).  After everyone left and I was still working on the door, I checked the master bedroom and found the mattress had been thrown on the floor next to their bed. I'm not sure if they were trying to hide it from me? Also the roommate for whom I was repairing the door requested a lock on the door, so my curious mind says he wants it to keep the kids out of his room. I'm at a loss for what to do since they've been great tenants so far, in that they have kept the house immaculately clean and organized and the kids seem well behaved, but it might be too soon to tell. My suspicion is that the boyfriend is staying in the master bed with my tenant, and his two kids are sleeping in the 3rd bedroom. Adding fuel to my suspicion, after I finished the door for the one tenant, they requested I install a door for the 3rd bedroom so the cats won't get in. In my mind, they wouldn't care if the cats got in unless someone was sleeping in that bedroom every night. 

I read through my contract, and my understanding is that I can prohibit guests from living in the home, but unless I do that and they continue keeping him, then I don't see that they are in breach of contract. I am using the Rentler.com rental agreement in case anyone is familiar with that. I am curious if anyone has had experience with this and could share. How should I handle this? On the one hand, they have kept the house very clean and I feel they will be good tenants and I don't want to start off with a confrontation. On the other hand, I definitely would not have been open to renting my house out to 3 adults and 2 children in a home that only has 1 tiny bathroom! And I can't help but feel like they were planning this from the beginning and perhaps were attempting to avoid a background check for the boyfriend?? In any case, as this is my very first rental, your input is GREATLY appreciated! Thank you!