Opposite sex children sharing a room

I am divorced and have 2 children ages 10 boy and 8 girl. I have a wonderful partner to whom i am not married to because neither one of
us wants to get married again. We live with him and his 15yr old son and we only have 3 bedrooms. My daughter and son share the biggest room
and his son has the smaller room because he is a teenager and needs his privacy. I have certain rules for my children to follow because of sharing a room.
My ex has expressed his disapproval at them sharing a room and says that it is illegal for them to share a room and inappropriate and has said
that he could take my children away from me for letting them share a room. I am a very overprotective mom and watch my kids carefully and they are
loved and taken very well care of in our home. Can he take my children away from me?

Thanks so much

He may file a motion and attempt to change the custodial arrangement, or seek that a judge intervene regarding the shared room, in my opinion this is not a reason to have your children removed from your custody.

can a judge change the custodial arrangement without me being involved? would we both have to go to court to prove our case?

No, you would have to have a hearing and both of you will be allowed the opportunity to present your case.