I use only one credit card and use it for everything. It gets paid off in full every month and has never carried a balance. We also opened a credit card for our son. He uses it for books and tuition and occasionally a meal or computer stuff.
My husband feels it's important to help our children become established with credit. Our 18 year old will be getting one soon.
Debit cards are risky for the exact reasons Daisygirl mentioned.