I think my roommate is giving her 2 year old too much melatonin

So basically my BIL and his baby mama moved into my house about four months ago with their 2 year old. My SO and I just trying to help them out because they are 20&21 and didn’t have anywhere else to go. We have noticed some things here and there that they do with their kid that we don’t agree with, but hey not my kid can’t really say anything. Well it has reached a point where I’m extremely concerned for her well being. They stay out with her at their friends house until 12/1am some nights, they started to give her 5-10mg of the melatonin tablets at the recommendation of their friend (they sit over there drinking all night so it’s easier if she’s asleep for them to do that). My SO and I have both said to them that it’s not good for her as they’ll give it to her around 8pm and then she’ll sleep until 1-3pm the next day. Well ever since we said something to them about them giving it to her they’ve been denying that they are. But then last night they came in around 930pm she was passed out over her dads shoulder, he made the comment “yeah she fell asleep over there” and took her to her room for bed. Well it is now almost 2pm and she still hasn’t woken up for the day yet. I’m just worried that they are going to overdose her on the melatonin and accidentally kill her. Are we just worrying too much? Should we try talking to them again? Should we get someone else involved? I just don’t know what to do now. Any advice is appreciated thank you.

***UPDATE****

So some have asked for more information, her dad is severely Autistic has anger issues and treats her mom like absolute garbage. Her mom does anything her dad tells her too and doesn’t question it to avoid fighting because he’s an ass. So if he tells her to give the kid melatonin then she does even if she doesn’t agree with it. On top of it, they don’t have health insurance. Can’t take the kid to the doctors because they don’t have insurance, I brought them paper work from Job and Family services so that way the mom could fill it out and I could turn it in for them but the dad found out and told her that HE would take care of it and that he didn’t want her to do it so they’ve gone three months with no food stamps,wic, or health insurance. Because he just won’t get it taken care of and she won’t go be hind his back and do it. So I’ve been sharing food with their kid to make sure she’s eating and drinking because they have no food or anything in my house for her and I’m not going to just let her go without. If I call CPS they’ll know it was me who did it as I’m the only one that knows they’ve been drugging her and how they live because they live with me. Would I care if they knew it was me? Sorta because they are in my home, it would make family gatherings hell especially if CPS did nothing which is what happens 60% of the time, and if CPS did nothing then her parents would just move in with their friend who told them to give it to her in the first place and she’d be in an even more sketchy environment all the time. I’ve spoken to my SO’s mom who is also the kids dads mom as he is my brother in law and she freaked out on them about it because this isn’t the first time they’ve done this. I guess they were giving it to her when she was less than a year old too. Stopped because his mom got involved and have now started it back up again. Also they say it’s melatonin, but it’s not a gummy like you all have talked about. They look like light purple tablets, like chewables, they put two of them into her cup after smashing them up. She drinks the cup and like 20 mins later she is passed out and NOTHING wakes that child until she’s ready to get up. I go in and check on her constantly because I’m afraid that she’s dead in there. The only reason I haven’t called CPS yet is because I feel like they aren’t going to do anything and then we won’t be able to watch her or be around her. Where right now we can make sure she eats, gets changed, gets a bath, and is in a safe environment while she is here. But if we report them and CPS does nothing then we won’t be able to do that and then they’ll definitely kill her. It’s a shitty situation all the way around.

So what can I do to make sure that when I call to report that they’ll actually do something to save her? She’s a sweet little girl (not here recently because of whatever they’ve been giving her has been making her extremely mean and agitated) and I don’t want anything to happen to her. But it’s getting to where I can’t just sit back anymore. Please help.

UPDATE NUMBER 2!!

Alright so obviously you guys aren’t understanding or have ever been in this situation before. CPS will not doing anything, said they can do a wellness check and depending on how that goes will depend on if they will open a case or not. I asked what they would be looking for during this wellness check and all they said was that there’s food in the house (which there is because I buy it so won’t fail that), if electric/water worked (again it does because it’s my house) and if the kid has any bruises or anything (she doesn’t) so to me it sounds like they aren’t going to do anything about it. The lady on the phone LAUGHED at me when I told her what was going on with them giving her sleeping meds and drugging her. Asked me if I had kids of my own and when I said no she told me that I wouldn’t understand until I had kids why you need to do that sometimes. So you’re advice has done nothing but probably make it to where we will never see our niece again unless it’s at her funeral. And I wasn’t worried about the family gatherings being weird. I’m worried that her parents are going to take her away and not let anyone see her to make sure she’s being taken care of. They won’t let anyone watch her. And yes they drink and drive with her in the vehicle. I’ve spoken to them multiple times about what they are doing and they don’t care. So now that CPS has laughed at me and blown me off what do I do?