First of all, I commend you for doing research on small pets especially when children are involved. Too many parents don't and just buy the first gerbil or hamster that catches their (or more often, their child's) eye (after all, they are traditional children's pets and have all kinds of commercial stuff made for them--much of it not actually good, so research on them is important too). My mom did research for my first pet, and he was a wonderful albino rat. For a responsible child, a couple of rats are nearly perfect pets, gentle, clean, affectionate, and patient with clumsy little hands. I would never go to put an age limit on pet ownership. (My first rat was given to me before kindergarten.)
If you have just finally given into your child's begging for a rat despite that you may have an aversion to them, please remember it is up to you as a parent to shoulder the responsibility, especially with a younger child, if the rat is to need extra care/finances or your child loses interest and neglects the animal. Remember, too, that rats do not live long. Most make it to two years old, some longer, and some never see their first birthday. If your child isn't ready to handle the whole spectrum of life and death, rats may not be right for the family yet. Also, they are not disposable pets. Though life for a rat is short, it still deserves a full and enjoyable life. Don't just "try one out." Rats bond to owners and their home quickly and really suffer from being shuffled around...
Make sure both you and your child understands the commitment (time, attention, finances...) and needs of rats. If you are not sure rats will fit into your family, try pet-sitting for someone with rats or watching a classroom pet over the holidays. Or visit a rat breeder/rescuer to see what a well-cared for rat is like. Ask lots of questions. Trust me, most of us LOVE it. (We simply can't shut up about our pets.) Even if you aren't all that enthusiastic about rats, you need to be involved to the point where you can take care of them in an emergency. If you cannot hold a rat (yet...), don't get one for your family. All pets are family pets, because no animal should have to suffer from neglect, and as a parent, you need to be able to take care of animals (maybe finding them a new home) when your child won't or can't.
I have a final word of caution. You will get attached to your child's or family's new pets. It happens all the time, whether a mom ends up just seeing them run to the cage door when she puts her child to bed or ends up carrying them on her shoulders while the kids are at camp, it can't be prevented. Then the kids move out and with them goes your excuse to keep rats around. Don't be embarassed. :) They aren't just children's pets. Rats are accomplished theives that have stolen many hearts. I have never appreciated mine as much as I do now, as an adult. Perhaps they keep me young. Odd how they can help a child grow up and an adult stay young... No rat owner has ever refered to them as "just a rat."
I wish you lots of happiness with the new members of the family!