We go with our kids and it is not a problem at all - never found kids running around the ship on Princess at all unlike some other cruise lines. Everyone gets a card and more for security reasons than anything else - we do not allow our children to put charges against the card which you can do so they can not spend anything. Towards the end of the cruise we will go to the shop and get anything that they need. Just authorise your credit card against the adult cards. You can also put some money US dollars against the kids cards and when they run out they cannot use it for purchases again. Not bad that it is in US dollars we have saved so much money this year - our bill was £300 less in May when we went because of the conversion rate.
You can buy lanyards from the shop.
hope this helps and have a nice time.