They claim that once you mold them to fit your teeth you can reuse them as many times as you want but I found this to be totally false. Once you pull them off of your teeth, when you try putting them back on, no matter how much pressure you apply, they gradually loosen within a few minutes and will work themselves loose until they fall off. Therefore, I would suggest if you want to use them for a party for example, put them on about a half hour prior to make sure they are molded well and secure, then don't mess with them unless necessary. They may be okay if taken out for a few minutes to eat or something and put right back in but I had taken mine out for several hours after I molded them before attempting to put them back in.

will they damage your teeth in any way.

As long as you do not have caps or veneers on your teeth, they would not damage normal teeth.

can this be used for a 9 year old even though the canines are baby teeth?

i would imagine so. i dont see why not. just when you mix the molding thing to the inside, put a lot in the false tooth before applying to the tooth.

Can be use for a 7 year old?

As long as you help the 7-year-old with the glue, to make sure the glue doesn't harden before they are put in the mouth, yes. I would caution the child not to eat with them on, so they don't swallow them.