I have a 5 year old DD and an eight year old DS. I just had three girls (thought there were going to be two) ages 6, 9 and 10. I didn't know the 6 year old was going to stamp and while my back was turned she started stamping. They were using my Mini Messages and Loveing Hearts. The 6 year old was very hard on my Mini Messages. AND she used it in black. She got it all over my blocks.
I am very particular about my stuff as well. But I learned a long time ago that it doesn't matter what age, they all will mess your stuff up. As I was cleaning them I just had to keep telling myself that it does not affect the quality of the image if it has ink on the wook. Luckily, I made a big enough mention about cleaning it before you use another color that no one made that mistake. The mom was here for the stamping part and was alot of help.
I have Crayon Kids, Kids at Play (I consider that my daughters... but if I ask real nice she'll let me use it), Pocket Fun, Girlfriends, Toy Box and Sporting Goods. The older goods like the flower and butterfly ones too.
Have fun. Way to get them started young. That can prolong the life of your business.