I have a 3 bedroom unit with 3 adult and from time to time, they have 2 toddlers that stay for a short period of time from time to time They also have 2 teenagers that does does the same thing what would be the best way to handle this
Before anything else, be careful how you frame this one, because familial status is a protected class under the Fair Housing Act. Any rule you write or any letter you send that is about children specifically, or that treats guests who are minors differently from guests who are adults, is the version of this that gets expensive. Everything below is neutral on purpose. I am not an attorney.
Start with the math, because I do not think you have an occupancy problem. Three adults in a 3 bedroom is well inside normal. The common starting point used in fair housing analysis is roughly two persons per bedroom, which would be six here, and that is a guideline rather than a hard rule, with local code governing. Chicago has its own occupancy provisions tied to floor area, so check those, but a 3 bedroom with three adults and intermittent kids is almost certainly not an overcrowding case.
So the real question is not how many people are in there. It is whether somebody has quietly moved in. Those are two different problems with two different answers, and your post does not tell me which one you have.
The cheapest first step is to ask. Not a notice, a conversation. Grandkids every other weekend is a completely different situation from a fourth adult and their children gradually becoming residents, and tenants will usually just tell you which it is. You lose nothing by asking and you learn the one fact that decides your next move.
If you want a rule going forward, the mechanism is the guest clause in the lease, and it should be written in nights, not in people. Something like: a guest is anyone staying no more than 14 consecutive nights, or no more than 30 nights in any 12 month period, and anyone staying beyond that must apply, be screened, and be added to the lease. That is neutral, it applies to a brother-in-law exactly as it applies to a teenager, and it is enforceable because it is about duration rather than about who the person is. Put it in at renewal for every unit you own, not as a one-off letter to this tenant.
Two practical notes.
If you pay water, trash or any utility on this unit, that is your actual early warning system. Headcount changes show up in consumption before they show up anywhere else, and a sustained step up is worth more than a neighbor's opinion about who is coming and going.
And on process, Chicago's RLTO covers most residential rentals in the city and sets its own notice requirements for curing a lease violation, with some exemptions including owner-occupied buildings of six units or fewer. Confirm whether your unit is covered before you send anything formal, because the notice you send under the wrong framework is the one that gets thrown out.
One last thing only if this happens to be a voucher unit. Unreported household members are a bigger deal there than in a conventional tenancy, because household composition is certified and it drives the voucher size. Different conversation, and it goes through the housing authority rather than through you.
I think the biggest question is how often the kids are actually staying there. Are we talking a couple weekends a month or several nights every week? Personally, I wouldn't be too concerned if they're just visiting periodically. If they're staying regularly, I would want to better understand the arrangement and make sure the lease accurately reflects who is living there. I would also be careful with how you approach this from a Fair Housing standpoint, especially if these are children of the adults living in the property. At the end of the day, if the tenants are paying rent, taking care of the property, and the occupancy is within applicable guidelines, I'm not sure there's much to gain by making an issue out of occasional visits.
How often are they actually staying over? If it's just occasional visits, I wouldn't make an issue out of it. If they're there most nights, that's different conversation about who's actually living in the unit. I'd clarify that first before deciding whether anything needs to be addressed under the lease.