I NEED SOME ADVICE PLEASE

I have a 5 year old daughter and a 2 year old son. My son currently sleeps in his sister's room because he doesn't want nothing to do with his room. My fiancee and I even went out and got a bunk bed for them. They share a room now which I don't mind. BED TIME is the worse with them. We tried to put our son in bed first since he's younger. He would scream, kick, throw the biggest fit ever until his sister comes up to bed. Then she starts. Now it's WW3 in my house at bed time. They would scream for me and I would go up and tuck them in for bed and give them a kiss goodnight but right when I leave to go downstairs they start to scream and throw a fit again. Some times it gets so bad that I'm scared someone is going to call the police because BOTH kids are screaming bloody murder. I just don't know what to do anymore. Last night got so bad that I couldn't help but bawl my eyes out, I don't know if its because I'm pregnant but I'm feel like I am failing as a mother. & that's the worse feeling I ever felt before. I love my kids more than anything but I am lost. I need some advice please.

If you don't have anything nice to say don't say it because I am asking for advice NOT to be criticized. Thank you.