I think it depends on the age of the child and how well they are able to understand what is going on. Luckily, my kids were mostly bald until past their first birthdays. The boys were over two by the time they had a “real” haircut. Neither of them cried, but the big challenge was convincing them to stay still. Sometimes it was easier to have them sit on my lap for the haircut until they were old enough to remember not to wiggle.
I remember telling them getting a haircut was easier than having fingernails clipped and just as painless. The lady who cut their hair was very patient and would show them that she wasn’t going to hurt them. She’d even let them play with a spray bottle and spray water all over the place while she cut their hair so they’d be occupied and sit still. No matter what you do, some very young children are going to be nervous because it’s a strange situation with a strange person. Someone I know used to cut her twin boys’ hair while they slept, but I have never been able to imagine how she managed it.
It might also help to read a book about getting a haircut, have the child watch you or an older sibling get a haircut first, or play “haircut” at home with a small comb, a spray bottle, and some fake plastic scissors (like the kind that usually comes with a kids doctor set). Of course, the drawback to that last suggestion is that the child could find some real scissors and do their own haircut.