You can do it, the TS credits are not split into child/adult forms. If you should/should not do it is up to your own moral code, everyone has different beliefs on what is right and what isn't. As far as I know some Disney CMs have suggest using children's TS meals for adults and just paying out of pocket for some of the child's TS meals.
So yes you can do it, it is up to you if you feel it is right or not.