Tenant using laundry before they should be...

[Robert P.]

So I am looking for a little feedback on what you would do. Here is the situation.

I just got new tenants in (they have been living there for about a month and a half. I have been nothing but attentive to all of their maintenance requests/etc. (nothing major, but they know I am someone who gets things fixed fast and correctly)

In the basement, I charge a monthly fee for any tenant who wants to bring in their own washer/dryer and hook it up to use. When they first moved in, they did not have a washer dryer, but have since purchased a set and told me they will begin paying the fee in July to use the laundry. 

Well, I just went by tonight to drop off a door I am replacing (for another tenant and put it in the basement for the time being) Low and behold, the dryer was running and the washer had wet towels in it. They were obviously using their washer/dryer before they should be in July. 

My question. Do I just straight up tell them, since you were using it, I will be charging you the fee for June. Or do I let it slide and pretend like I never saw it? I think since we are still establishing our tenant/landlord relationship that this is a perfect example to show them that they can't walk all over me and do things without my permission. In the short time they have lived there, I have responded extremely quick with all of their maintenance requests. (some examples, bathroom fan/vent was not working great....so I replaced it, ripped screens..replaced them, water leaking out of glass sliding shower doors....fixed that, I even helped them hang some shelves they couldn't get into the drywall!) I am also in the process of getting a house meter installed since the common areas are hooked up to their panel. To make up for this, I have paid their 1st month's electric and was planning on doing the same for the next bill since it just got fixed this past week. I PAID THE ENTIRE BILL! My thing is, we are still getting to "get a feel for each other" and I don't want them mistaking my kindness for weakness. 

So what would you do? Let it slide, or tell them...since you used it, you have to pay the fee. I want to set the tone, but also don't want to be the bad guy. Thoughts???