With some providers, you have to have this with GP & Prescriptions, others it can be added to the base hospital plan.
Typically medical insurers, who offer this add on, will allocate a certain value per person per policy year for this benefit. Like GP's & Prescriptions, it is designed to address early intervention rather than cover the big stuff.
Things like your dental checks and maybe a filling every year, but not a whole lot more.
With optical it is a regular eye exam and updated lenses or contribution to lenses, if yours are the expensive kind.
If you are looking for a solution to your child's braces later on, unfortunately this benefit does not extend to periodontal and orthodontic treatments. Nor does it cover laser eye surgery. Neither situation can be insured for currently.
Like GP's & Prescriptions, this is an area where people can easily end up paying more than they ever use or claim. Weighing up what you need and understanding what you get for your premium is important.