The voucher does not guarantee you a PS (reservation). You will still need to call 1-407-WDW-DINE in order to make a ressie for your desired date and time.
Since most character breakfasts are buffet, you cannot share vouchers. Each member of your party will need to pay for their meal. If the meal is à la carte, you may be able to share vouchers but I have never tried it.
In general, the children's breakfast vouchers are not a good deal. You would be just as well off paying for their meal out of your pocket unless you have gotten them as part of a package.