I agree! There is too much confusion regarding the kids! It's been established that Ranbir, Nishi, and Arav are Ragini-Neil's kids as we saw their faces in flashback. I'm so confused about Suhani and Agam. Suhani seems to be Ragini-Neil's daughter. But it's weird the way Ragini reacted in the promo when Neil and Suhani/Ranbir show up, she singled out Suhani seemingly negatively. Someone mentioned that Agam was Pam son, which would make sense as to why Suhani would get a car before Agam, if Agam is older than her. But Agam still calls Neil 'dad' and stuff so its confusing!!!
As for the keeping kids separate, idk. I thought it was like a deal between Neil and Ragini. Each would keep certain kids and keep each other separate. They wouldn't have contacts between the kids, themselves, and between themselves and other kids. So basically, for Ragini, the only kids she had were Nishi and Arav. And for Neil, he only had the 3(?) other kids. Idk. It's weird.
As for calling Nishi 'woh ladki' and not 'meri beti.' Well, he's only just met her after years. He still doesn't want to get involved in their lives. It goes against some agreement Ragini mentioned. So calling Nishi 'woh ladki' is probably Neil's way of stopping himself from becoming emotional in regards to Nishi. Although this will all change when he comes to India to participate in the marriage, I think.