Nichol, turtles have long lives and are not the easiest of pets to take care of. Kids grow up, interests change, move, etc. I would recommend that they get a pet with a shorter life span that is easier to care for.
If they are into reptiles (as I was and still am!) a Bearded Dragon or Tegu is a much better choice than a turtle. You need to consider your needs in the years to come too, so if you are not into reptiles consider getting them a pet dog or cat.
If they really want something exotic, there is a writer here on Hubpages that has a lot of good advice. Her name is Melissa A Smith if you want to check out her profile page. If you want any advice after you have chosen something for them you can leave me a not on my profile page email.
Good luck to you--and the kids, of course,