Asking for rent

My wife’s 30 year old “daughter” (someone who came into her life at 13 because she was friends with my wife’s bio daughter) moved in with us 4 weeks ago with her 35 year old partner. Their intention is to save money to get out of debt and a down payment for a car. They are both terrible with finances, spend an obscene amount of money on food and basically travel to work (they’re photographers who will take gigs just to get travel expenses paid). There’s a whole lot wrong with this but I felt obligated to say yes to them moving in and suggested they pay $1000 a month in rent. My wife didn’t want to charge them anything so we compromised on $600. We met with both of them prior to them moving in where I told them I had no intentions of supporting 30some year old adults. We told them the expectation of what we would want for rent and they agreed. They have now lived here exactly 4 weeks and haven’t offered up a single dime or acknowledged anything to do with rent, not any intention to pay… They were previously paying $2300 month for their apartment so idk why they wouldn’t have the money to pay us or if they didn’t take us seriously…

I’m at the point where I’m going to do one of two things come April 1st. I’m either going to send them a Venmo request for past due plus April or I’m going to buy an invitation, stating something like “you are cordially invited to pay rent” with a break down of what they owe and stick it on their door. I’m just floored that two capable adults who are now living under my roof, haven’t so much as thought to pay rent that they agreed to.