Yeah, kids are totally allowed! Family cosplay is encouraged. There are some really cute child cosplays that can be done. Lots of people like to opt for child-friendly things like Pikachu (or any pokemon for that matter), animals, or simply a fuzzy jumper with a catears headband. There are some nice possibilities with that. But, for your question, yes, kids are most definitely allowed. ^_^
And for the other question, you would probably need a sewing machine to sew the pleather into the pattern for pants. You could do it by hand, and as long as you take your time and do a tight sewing job, that would probably work just as well. And if that doesn't seem satisfactory, there are people on here who make costumes for a living, or at least have sewing machines that I am sure they wouldn't mind lending. (I speak for others because I have no sewing machine of my own, heh). Anyway, best of fortune to you with this costume!