We just got our first kitten - my kids are 6 and 9 yrs. I think my 6 yr old is just about old enough to understand about being gentle but she still tries to pick her up too much and has been scratched a fair bit when kitten is playing with her.
I'd worry about your younger one just cos I think even at 5 they need supervising and its not always possible.. We got ours from cats protection and they said we were on the cusp of being accepted due to ages of our kids.
I think it's a good idea, Children can get a lot of happiness from animals, but don't be surprised if the cat decides to up and leave if your little ones are loud!
This happened with one of ours, he left his previous owners as they had 2 young girls who were very loud and he left to find a quieter life! Teach them how to handle the kitten correctly and not to be 'grabby' with the kitten, cats really don't like it and it can end up with the children being scratched. It's not the cats fault, I wouldn't want to be man handled either, it's often that the child hasnt been taught how to approach/handle the cat.
Cats are very different to dogs, everything is on their terms, which young children often don't understand, but if you take it slowly and explain to your kids about the right approach to the kitten/cat, I think you'll get a lot of pleasure from a furry addition to your family.