A bearded dragon is probably one of the best to start out with for young children. They are large and hearty, but one of the more social reptiles, so won't be as adverse to handling.
A couple of precautionaries, though. Remember that with any reptile there is a chance it can pass on salmonella. Make sure your kids don't ever kiss the dragon, or handle it and then rub their eyes or put their fingers in their mouths. Keeping the cage very clean and making sure that you and anyone who handle the dragons thoroughly wash their hands reduces the risk of salmonella.
You may want to check with the kids' doctor to see what age salmonella becomes less a risk. For you and probably the 5-year-old it won't be too bad, but it may be a little more serious for a 2-year-old. I don't know, so it's better to check with a professional who can tell you definitively. It could be a matter of just not letting them handle it for a little while.
Other than that, I think it would make a great learning experience for them, and I think a Bearded Dragon makes a great starter reptile.
Here's a caresheet for you to look over:
http://reptileguides.herpcenter.com/...-t15.html?t=15