Turtles (and tortoises in particular) are not really good pets for households with small children. Kids like to hold and touch them, and most turtles don't really like to be touched or held. Herps in general are always a risk for Salmonella poisoning, there are way too many instances of it infecting small children from something as innocuous as merely washing the turtle's food bowls in the kitchen sink. There are other, more insidious zoonotics, like Cryptosporidium to consider when bringing a herp into a household with a child, especially one as young as 18 mos.
I kept and bred tortoises for years, and still do at work. If someone thinks they are easy animals to care for, I think there is every possiblility that they aren't taking proper care of them. Especially in temperate areas of the US. Tortoises in particular have very specific dietary and lighting requirements that most people are unable or unwilling to meet. Not meeting those need results in deformities of the bones and shell, and I have seen some horrific deformities in captive tortioses.
That said, if I was going to pick one species for your circumstances, I'd go with a Reeve's Turtle (Chinemys reevesii
). They can be found as captive raised youngsters, are relatively easy to keep and can be very interactive with their keepers. They are small, usually around 5-6" in captivity and semi-aquatic. You can comfortably keep an adult or two in a 55 gal aquarium ...which is doable for most people. A youngster is going to need something smaller to start out with, and gradually move up to the bigger tank. But it's never going to be an animal you want to take out and handle.
If you absolutely have to have herp, I'd recommend a Bearded Dragon or Leopard Gecko. They can be held, are pretty easy to keep and more suitable for the family you described.
Here are a couple of Turtle/Tort sites you can check out:
California Turtle and Tortoise Club
The Tortoise Trust
Here is a good all around Herp site that can help you do some research:
Mellissa Kaplan's Herp Care Site