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Our dentists didn't have DD come in for a check until she was 1; and our optometrist said he doesn't have kids come in until they're 5 (they have to be able to understand/respond to the vision tests). So I don't think it's necessary to have them on dental/vision insurance before that.

I work in insurance (medical/dental/vision). Unless your company is paying towards the premium of your dental plan, I would not sign up until LO is around 2. Dental insurance is basically pre-paying the cost over 12 months and usually evens out over the course of a year if you don't have any work done and just have preventive care. It's probably cheaper to just take your LO to a pediatric dentist and pay out of pocket.
Of course if your company pays 100% of the cost, then there's no cost for you, so you could definitely do it sooner!

I agree that vision insurance isn't needed until they are older, around 5. Also, same concept as dental... You just prorate the cost of services over a year vs paying it all upfront with vision insurance.