Is cutting someones hair illegal without their consent?

Yes, in most civilized societies this could be considered battery. Even threatening to cut another person's hair could be considered an assault if it appears you are capable and likely to carry out the threat, ie, you are wielding a pair of scissors or a knife for the purpose of carrying out the threat, and especially if you are making the threat in a menacing fashion.

On the other hand if you have given your dependent child a haircut, whether they like it or not, you will probably not need to worry much...unless, of course you have used undue force, or something beyond acceptable parental authority.