I can only relate my DD's experiences.
Her first two dentist appointments were partly with me and partly without me. I held her on my lap during the digital camera inspection of her teeth (age 4; no xrays) and I went into the exam room with her to meet the hygienist and dentist. Then, when she was comfortable, I had my appt in a nearby room. She loved the staff and knew where I was and was very happy.
Different dentists since then have different procedures. In all cases, she was fine if I was nearby and she knew she could call out for me or see me. The best set-up was when we were in exam rooms next to each other with a big glass window between us. She could see me; I could see her. The dentist came into my exam room to talk with me about DD's exam while my teeth were being cleaned. DD saw everything from her chair in her room.
At her most recent dentist appt (age 8.5), we were both new patients to a holistic dental office. When they called her back, I automatically stood up to walk back there with her. Not to hover, but to meet the people who were going to be caring for my DD's teeth. The staff person explained nicely but firmly that she needed to establish rapport with her patient. I kindly explained that was fine, but I'd like to meet her and the hygienist and dentist before sending my daughter to the back of an unfamiliar office. It all worked out, but I did have to assert myself. DD's appt was finished first and she came into my exam room and sat on the floor quietly doing math on a clipboard instead of going to the waiting room. I have a feeling DD asserted herself in that situation.

(I had already written out the math problems in the waiting room when we first arrived, at DD's request.) The staff seemed used to parents and kids doing what the staff requested without question, but didn't have a problem with us doing it slightly different. We were all polite and courteous towards one another and it worked out well.