Bring them from about age two, and bring them to your own appointment before that. A child who has watched a parent lie in the chair twice already is a different proposition from one who has been brought in at six because something hurts.
The first appointment is deliberately uneventful: sit in the chair, make it go up and down, count the teeth with a mirror, and leave. No probing, no polishing, no x-rays. If that goes well the next one adds a polish, and the one after that adds fluoride varnish. We are not in a hurry.
Fluoride varnish twice a year from age three, and fissure sealants on the first permanent molars at around six or seven, do more to prevent fillings than anything else available. Both are quick, neither needs anaesthetic, and sealants are £45 a tooth.