Basically, enroll them slightly before their services are needed.
You're right: There's no reason for her to have dental/vision coverage now when there's no way for it to be used.
However, some plans have "vesting" times, where the individual must be covered for a particular length of time before being eligible for coverage by some portions of the plan. (e.g. Must be covered for 2 years before eligible to have braces or dental surgery covered.) Find out what these are, decide what your children are likely to need and enroll appropriately.
As HedgeMage so wisely noted: What is "medically needed" and what is "needed to do X" may be 2 different things. Some schools or organizations that you'd like to enroll your child into may require tests or screenings beyond common guidelines.
Finally, be mindful of those enrollment windows and don't miss them. If you or your wife are at all forgetful in any way, the peace of mind might be worth the incremental cost. I have a memory like a sieve -- getting it all in at once, even if the services were of no use at the time, would be better for me than forgetting to enroll them and have to pay out of pocket for services for a year (or possibly more, if anything other than routine visits were required and not covered).
Addendum: If you have access to a Flexible Spending Account (FSA) at your workplace, enroll if you haven't already. You'll be going to the doctors quite a few times for the first few years for both well-baby and sick-baby vists, and if you have co-pays on visits and prescriptions, they can be quite a chunk of change.