I have a long, messy history with cps. I'm not going to read the other replies because all the "they're just there to help, don't worry" attitudes usually eat me alive. My best advice is to not let them in. Have a sit with them out front. Chat a little bit. Answer what you feel is necessary. I would not sign anything giving them permission to speak with the kids doctors, etc. Let the kids come outside with you and say hi. Inform them of the situation going on with your inlaws.
With out a warrant (which they won't get, not matter what they say) they have no right to be in your house. They may even come back with the police and try to get in your house that way. If there's no warrant, it's the same thing. Politely and firmly tell them that you're not comfortable with that for now.
They often get bored and go away after the front porch chat. Especially if you don't sign anything giving them permission to probe further. I mean you could play nice, and let them in, and hope for the best (the whole mentality of having nothing to hide). I've seen that really work for some people and really screw others over.
I also would not agree to meet with them at their office. Those doors lock from the inside usually and if they don't want you leaving with your kids, you won't.
I would, if I were you make an appointment for the kids at a peds office. Just pick a doc at random if you want and pick a date kind of far away if you don't really want to go. Because they are
going to bug you about the kids doctor...it's like one of their favorite subjects. And it helps to be able to say that there is a check up coming up for the kids soon (even if you plan to cancel it, they don't need to know that). Or really go and get the check up if that'll make you feel better. Again, I don't recommend giving them permission to speak with your kids doctor (even just to confirm that you have an appointment).
I'd also buy beds or whatever big things that will make you feel better to have "just in case". Cps is a bit obsessed about beds. I still don't recommend letting them in.
Hopefully all will be well and it won't be a big deal, it usually isn't. But people can and do lose their kids to cps over nothing. No abuse or neglect is even necessary. Just the potential
for abuse or neglect needs to be there. And that's up to anyone's freaking interpretation.
But I'm not going to argue about any of what I just said on this site. People here are usually either very pro or anti cps and it makes it hard to have a productive conversation.
Hope things work out for you OP. Sorry about the inlaws.